Rating solo performance in a BG is not hard at all, other games have done it with complete success(and I wasn't just talking about solo to begin with, just casual, it's a completely different thing). As long as you are doing something they can make it to where you get points(or however they wanted to implement it) towards your rating. Winning BG's, doing damage, doing healing, CC, getting damage done to you, CC'd etc... it can all be added in and it wouldn't be difficult for a programmer to figure out how to do it once they decided how to do it. Blizz never did it because it wasn't a part of their original vision for rated BG's is all. Once they get a vision on how or what they want to do, they are not going to deviate from it and make it they way they wanted. More power to them, it's their game after all. But in doing so they show the disconnect that they have with their playerbase.

Having the argument about defending certain objectives doesn't really fly however as well, I have seen numerous people 'defending' a mine, or an obstacle in AB, or a flag in AV to the point where they weren't defending anything. They just sit there, do nothing, only to let the opposing faction to take the objective anyway, and when no one is around(if then) then the re-cap. It is just a new strategy for them to go AFK without staying in the starting area and getting reported.

It would not be as hard as you might think to implement such things, some companies may have someone come out and tell you that, but it isn't true. The truth is that they just won't go away from their original plan, and they really can't in that respect otherwise it would take forever to come out with anything if they had to start doing something else mid phase of the plan. The people protecting flag room in WSG won't have to worry(unless they do nothing) because there will always be someone going for the flag, Protecting the leader in AV, no problem there either again, as long as you are doing something.