It basically goes like this: Perhaps the reason that Terran is slightly favored in win-rates vs Protoss, and more so with Zerg, is because Protoss players do not make use of all three spell-caster units effectively.
In PvT, the Arbiter is obviously the most heavily used spell-caster unit, with the High Templar being used on occasion to defend expansions or tag along with the larger army of Zealots/Dragoons. But one unit is exceedingly rare in any Protoss matchup, the Dark Archon.
In late game PvT, the existence of a Science Vessel is very detrimental to the larger Protoss army, mainly because of the ability to EMP Arbiters and render the cloaking field null. HOWEVER, with a Dark Archon and a high energy Science Vessel, a Protoss player could reasonably kill one outright and effectively give his Arbiters more energy, assuming they would have been EMP-ed otherwise.
The ultimate ability of the Dark Archon, Mind Control, is difficult to use effectively because it is so energy-intensive, and one would need multiple Dark Archons and thus plenty of supply to Mind Control just a few units.
YET, Mind Control does not need to be used frequently. We can think of it in this hypothetical situation as a proxy Zealot Bomb or even a proto-Spawn Broodlings. Imagine Mind Controlling a Vulture near a nest of spider mines. One could effectively destroy an entire gang of Vultures, paving the way for Zealots. With enough Dark Archons, a Protoss player can cause Tanks to fire upon one another. Or even steal a few for himself after casting stasis on a smaller group.
What do you guys think? Am I forcing the Dark Archon where he dosen't belong? Or am I onto something here?