Quaternions might have great benefits for key-framing rotation (no gimbal lock and so on) , but I have to admit that when it comes to the 2D graph editor I don't stand a change of working out what effect of grabbing the handle of a key on a Quat's W channel will have on my animation. Generally I'll adjust the key in the 3D view and leave the bezier handles well alone set to auto (or vector if I want a snap) because independently changing any of the W, X, Y, or Z channels is unlikely to yield any great improvements for my animation!
Today I expanded my add-on to draw the handles for Quaternion and Euler rotations, and to show the path of the tail of the bone (which is probably what we want to track when we're adjusting rotations). The timeslide tool still works but I need to do a little more coding (and get my head around what a normalized quat is) before grabbing the bezier handles in the 3D view is possible.
EDIT: http://www.pasteall.org/19807/python - now with quaternion and euler editing! Editing preview shown below. At 1500 lines it outweighs the mushroomer, the crowd sim and the autowalker as my new biggest project... not bad for 3 days coding!