This post is way off! In competitive pvp tanks are very important for protecting your allies, especially healers. Usually a Guardian/Jugg fills that role, but I've seen Shadows do it all right. True vanguard tanks are exceptionally rare (because everyone is playing assault).
You seem to be thinking of tanking in pvp as a solo job instead of a team support role. In other words, speccing tank so you last longer and don't die as often. That's not the right way to think of it at all.
I was actually responding to a post claiming that guardian tanks were somehow OP in PvP, which they are absolutely not. I actually agree with you - tanks are not useless in PvP - but they're often not highly valued by players because they don't do well in PvP combat. Their primary virtues are guarding the healer and being able to stick around a little longer to hit interrupts and stuns when defending an objective point. I think most groups would rather than extra healer or dps instead, since they can both defend and attack.