I agree with the poster above. Everything an ST PT can do, a Tankasin can do better, at the moment. Pyro, on the other hand, has a rather effective, if a bit straightforward playstyle.
In PvP a ST is not the best tank due to the fact ST rely (almost exclusively) on passive migrations to mitigate damage. Tech and force attacks completely ignore this type of "protection". In PvE ST are just as "good" as any other tank with comparable gear. Now each tank class has its advantages and disadvantages so I'm not saying they are all exactly equal. One of the big advantages of the ST is its increased situational awareness that comes from less time managing active damage migration rotations.