Quats can interpolate smoothly between any orientations. Eulers… not so much. It’s not noticeably over smaller rotations but the different interpolation is very noticeable if there are significant rotations in all 3 axes.
Theodox has half (or possibly more than half) of the answer. The other factor is that quaternions allow you to overcome gimbal locked joints. Our animators like to key each channel individually and to be able to shift and scale channels around freely, so adding a quaternion layer would probably just piss them off.
When new to Maya I couldn’t understand why rotation interpolation would wobble all over the place. Then I discovered “euler filter” and never worried about gimbal lock again.
Though an Euler filter can solve a lot of problems, a determined animator can fubar anything with enough patience. What I’ve been doing lately is creating a layer of 3 parallel controllers (XYZ) in a layer directly above the primary controller for any controller that the animators thinks they will want to rotate a lot (hands in the case of some heroes, COG/pelvis for others). It doesn’t solve the issue entirely but it certainly makes it much more difficult for them to gimbal lock any certain controller, and it’s easier for them to back out of the gimbal lock if it does occur.