Having groups that cuts through layers without disturbing them might be pretty useful, actually. You could group things like wheels of a car, for example, where two are behind and two are in front of the chasis, and be able to move them around as a group while keeping chasis layer in the middle. Or perhaps be able to group hair, which have some parts in the front and some parts in the back of the face. Photoimpact groups objects this way (groups objects without disturbing the layers), and I often find it handy.