The RIFT soul system was interesting to me for a little bit, but then I saw the terrible difficulty they were having balancing the souls. Every class forum I went to had people complaining because every patch seemed just ping-pong around which builds were top dps. At end game there would be 1 maybe 2 viable dps builds the rest would just be too poor and because anyone could just use the top builds, everyone was expected to do so.
I don't want that to happen to swtor. By having AC as an important, unchangeable decision it forces Bioware to do the work in balancing 8 classes instead of 4. From my "history" in MMOs I've seen that the community as a whole is a lot more vocal when an entire class is weak instead of just one spec. If you let Bioware get away with having only 1 viable dps spec per base class it will make the game incredibly shallow.