There has been an ongoing discussion/debate about whether accelerometers are actually useful for attitude estimation on flying rotorcraft (quadrotors, in particular). The simple, one sentence answer is "yes, they are."
However, the accelerometer measurements are non-intuitive. If the estimation algorithm uses the accelerometers incorrectly (which is, incidentally, the norm on hobby and even some professional aircraft) then there will be significant errors.
There is some good literature available now that describes the problem. Martin and Salaun published a very good paper describing quadrotor flight dynamics. I also just finished my MS thesis, wherein I address the subject in detail.
In my thesis (the last link), Chapter 5 is what you'll want to look at.