periodic can be disspelled by sorc/sage, no idea about what other classes can.
assassin/shadow can become immuum which also disspelled the periodic dmg.
Besides this has been since the day swtor started and still people can get to the last door in no time so... it depends on your team and since there are so many people complaining about there being so many sorc/sages who can disspel I dont see how periodic is an problem.