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This unit which has received nothing but hate is probably the most widely regarded as a useless unit. Weirdly enough however this unit is the one I have the most to talk about. Many people have complained that due to the fact that the Replicant costs more than almost every unit it replicates that it must be useless. This is an EXTREMELY shortsighted analysis, as if the toss would simply use units as part of their A-Move deathball. What makes this unit so powerful is the ability to utilize their opponents SPELLCASTERS against them in combination with the additional Protoss spellcasters and units. Basically this one unit means that the Protoss army has at it's disposal every single spellcaster in the game.
vs. Terran: (Credit to Gyro_SC2's thread Replicant are TvP BIOCounter?)
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The utility of the Replicant is almost entirely based on one simple fact: If the Terran gets their EMP's off, they win, if they don't, they lose. In almost any high level game you will find that this is the case.
Toss being able to have Ghosts now completely throws off the balance of that engagement. The main reason is that the RepGhost will have cloak right from the start. Generally Terran players will have their Ghosts on a separate hot-key. Now Toss has the ability to send a cloaked Ghost in prior to an engagement and disable all of the Terran's Ghosts (similar to the NP self-EMP maneuver) or the Terran Medivacs. If the Protoss is able to land just one such EMP, their chances of winning the next battle increase significantly. The main counter-argument on Gyro_SC2's thread is that Terran usually scan before proceeding so it wouldn't work. However this argument rests on the ridiculous premise that Terran is ALWAYS scanning in front of their army and that the RepGhost couldn't approach from...oh let's say...the side (basically a really dumb counter argument). The fact of the matter is, the only way to prevent this would be for the Terran to have detection in the form of a Raven.
The problem now is that Terran would have to fight a Toss Deathball which now includes fully upgraded Ravens. I really hope I don't need to explain to you why Hunter-Seeker missiles and ESPECIALLY PDD (OMFG) would be extremely useful against a marauder-heavy bio-ball and vikings which desperately need to kill your collosus.
So basically with this one unit Terran's options are either A) Allow their Ghosts to be EMP'd prior to the engagement, or B) fight a Toss deathball that now includes Hunter Seeker missiles and a PDD.
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Ahhhh where to begin? This is the one match-up in which most people seemed to think the Replicant would be MOST useless, however I think this one is where they would most shine.
Basically when you're looking at the PvZ matchup it's vital that A) the Zerg is able to control the collosus count, and B) the Zerg must set up good surrounds/flanks in order to defeat the deathball. Both of these functions are directly countered by the Infestor's fungal growth. Now Toss has the ability to fungal growth corruptors and stop them from target-firing their collosus. They can also now include fungal growth with FF when zoning out Roaches. Being able to cut a Zerg roach-ball in half with a single spell will be very bad for Zergs...
Another small idea (which I believe was originally said by somebody as a joke to mock Replicants in PvZ) is using the Replicant to become a queen and then creating Creep Tumors. While obviously the Protoss shouldn't help the Zerg spread creep, the obvious use would be to send tumors back towards to Zerg's base. Basically each tumor becomes a stationary observer that the Protoss can use to gain excellent vision of the Zerg's entire side of the map. Of course, in order to stop you from doing this the Zerg would need to get Vipers for detection...
Which now means that the Toss player has Vipers :D. More importantly, they have Dark Swarm (Blinding Cloud w/e). In an engagement in which all the Zerg's units have range 1, zealots and archons have a SIGNIFICANT increase in power. Imagine combining that ability with force fields (Shudder...).
In conclusion, the Replicant has excellent uses both vs. Zerg and Terran. Both enemy races now must contend with the fact that any spellcasting unit they choose to get will also now be included, FULLY UPGRADED, in their opponents arsenal. How at any given time people believed this unit to be useless is so far beyond me that I've nearly lost all faith in the general communities ability to think critically. It's like the best people could come up with is turning the damned thing into a maurader...
But yeah, totally useless PvP :p.
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Another hated unit. While it's true that it may not work well as a direct counter to mutas, I feel that it has excellent synergy with the Toss deathball.
The typical anti-collosus deathball strategy by Terrans and Zergs is to have Vikings/Corrupters destroy the Colossus before they are able to fry the Terrans/Zergs entire ground army with their ridiculous DPS. In order to do this Terrans and Zergs typically have to have their entire AA ball on one hotkey shift-clicking the collosus. This typically forces the entire group into close proximity while they engage the individual targets (much more with corruptors than vikings). Therefore the addition of just two tempests in the Protoss deathball means that any vikings or corruptors will be taking ENORMOUS splash damage while they rush to engage the collosus. If the Terran or Zerg opts to magic box with their flying AA then there's simply no way that they will be able to target fire the Collosus down quickly enough to save their ground army. The Tempest provides extremely effective answer to Terran and Zerg air answers to Collosus. This will also allow the Protoss to cut down on their stalker count as they won't require the AA to protect the Collosus and now focus on a heavier zealot/sentry composition which is much better vs. Bio/Roach.
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Less hated than the Tempest, more hated than the Replicant, this is another unit which falls into the category of "The fact that people can't think of a single good thing to do with this unit hurts me." While it's probably a bit too expensive at this point I feel that it's abilities are extremely underrated.
I really don't understand why people are so down on the economic harass of this unit. Of course the response is "LOL JUST KILL MINERAL GOOP NO PROBLEM EZZZZZZ", to which the obvious question is why doesn't the logic of "well if my opponent tries to harass me with X, then I'll just stop it" doesn't always work for X=hellions/banshees/DTs/Infestors/Lings/Mutas etc. And while these forms of harass have the advantage of killing workers, the advantage of the Oracle's spell is you don't get a combat warning. If you don't notice it happen, the effects of having an entire mineral line out of action add up extremely quickly. The other thing to consider is that this spell could allow you to kill workers much easier in a harass. If you lock down every mineral patch but one then the entire worker line would group up on one patch. Now imagine you also brought along a High Templar or an Archon in a Warp Prism...
The Oracles building Corruption/Lockdown ability is another extremely useful one that I'm surprised doesn't get much love. The harass ability when combined with DTs is the most obvious application, and while overseer corruption didn't always get the usage is probably should have the fact is that often that would result in the loss of an overseer. An oracle is as fast as a pheonix therefore it's ability to get in and get out is far superior.
And finally, this unit allows you to see exactly what technology is in production at any given time. Being able to scout Overlord drops could be quite useful in a PvZ I think.
The point of this long rant is this: Just because you can't A-Move it doesn't make it useless. As with every unit in the game, you have to consider how these units can be used in conjunction with other units in the players arsenal. Now as none of us has had any extended playtime with this game nobody can know for sure what new strategies will evolve from these new units. However it makes me sad when unimaginative people offhand declare these units to be useless. You should all be better than that.