I don't think that there will be a certain class that will be called on to do just a certain task. From what I have played of the Alpha and Beta, the class you choose to play will be just as useful as any other class in the Raids. I have read no where that Bungie has said you must have certain classes to complete the Raids. Of course, at time certain classes may be more useful in some situations. For example, say your in a group and your are getting rushed my a bunch of the Hive melee things (I forget their names), then a Titan with his ground pound would be of great benefit, or if you have some enemies at a distant that are really giving the group a tough time, then a Warlock would come in handy with his Super ability and with the Hunter same thing with his Golden Gun to help take out a sub boss that has a lot of health. But with that said, I also feel like it is very important for a Raid Party to have some of each class and some that are spec'ed out as different sub-classes. I think that way the Raid party stands a much better chance of finishing the Raid.