They have unique story arcs and missions so there is a benefit to actually playing them since each of the epic ATs are two different game play expierences.
They seemed to on the test server. However, I've rolled one on live, and find myself doing the very same Vahz missions I've done with virtually every other AT I've played (to the point where the Peacebringer contact is mouthing the same dialogue I've heard from all the other contacts).
Unique content? Haven't seen it yet.