Tri-Disaster is purify and you're done. Tri-Disaster goes on CD, the SMN has to find another way to bring your life down low enough for Painflare and/or Deathflare to matter. Of course you may get Fester'd before you can purify it but thats nothing Recuperate can't fix. If you're around a healer? Great! You're gonna live! If you ever die solely by a SMN's "easy bake" combo, then you're definitely overextending a bit much.

DoTs stacking has never been a problem in PvP and I don't think it is a problem at this moment either. Kisada kind of pointed it all out how to deal with DoTs. You either Purify it or have your Healer LB, it'll remove all detrimental effects OR have your Tank LB so you're taking less dmg per tick(not sure on this one, may need some testing/confirmation).

As any DoW class, you have access to Recuperate/Purify that helps against DoTs. If you can cross class Second Wind even better. If you've got a healer nearby? Great! You're gonna live! (Saying it again)

If you're a DoM class, you have access to Sprint/Physick/Purify.

DoTs take time to kill you, if you know when to back off and get yourself healed up, DoTs aren't anything to worry about.