I've noticed ESPECIALLY in TvZ that tanks don't actually serve a purpose in my army other than to act as a gas sink that could better be spent on double upgrades for bio. Tanks are great against banelings, but so are conc shell marauders forming a wall to block the banelings from getting to your marines and critically, marauders with medivac support simply do not die to fungals. At all. Go ahead and fungal my marauders zerg, once you run out of energy I'll flood in with my 3/3 marines and kill you.
I spent so much time learning to split my army to mitigate splash damage that I find I don't actually need tanks at all and if I build them and I can't win with a mid-game timing push then they actually become a liability when zerg goes broodlords. If I go marine marauder then admittedly the marauders are equally useless at fighting the broodlords directly but there's a very critical difference. I can split them away from my army and counter attack far flung expansions with marauder drops, something that just doesn't work with tanks because of that pesky siege-up time. Also my marauders are 3,3 and can stim where my tanks, if I'm lucky are 1,0. 0,0 if I've been under a lot of pressure.
Generally my problem seems to be this, that tanks are admittedly great in large numbers. 6-10 tanks is a truly awesome thing to have in your army, but by the time I've actually amassed that many, zerg has a better flying version that shoots zerglings and I cry terran tears, cursing blizzard for nerfing my beautiful ghosts... Well not really, but you get the idea.
I never built tanks seriously in TvP, messed around with 1,1,1 builds, decided I liked bio better and never looked back, so that just leaves TvT where it's kind of a toss up on the map. If it's a map where I MUST push or hold middle control, I get some tanks because I feel you need to, but I'd dearly love to not have to. Marines though, you do need some kind of splash to fight them or they swarm you so it's either tanks or HSM. Seeing as I have a factory sitting there anyway, it's a lot easier to just get a few tanks with my bio army, but I feel like I want to get 3,3 ups on my bio before I do.
Anyways, I guess that's it, I feel like marauders do the siege tank's job better, plus they share upgrades with marines. I also see this trend a lot in pro matches too, where terrans are increasingly reducing their tank counts or cutting them all together from any strategy that isn't an all-in timing attack of some description.
Anyways, discuss, I suppose.