Maybe I was a little hasty. Crucially when it comes to keys, animators need to know the reference a value exists in – and that, it shouldn’t change. For example if I animate a guy jumping from one platform to another – the curves of the jumping, moving from and to the platforms shouldn’t pop erratically to allow its reference space to change. More over the the space the values are relative to should be transformed.
Offsets
The offset is a great mechanism for retaining space – it wouldn’t be a good mechanism for FK/IK systems because your trying to force the IK/FK into its opposite – i.e you want its value to change. Can we form a rule here? When you want the value to change directly transform the object, when you want to retain the value transform the objects parent space.
But..
You still need to allow this approach to be understood by the animator. The offset needs to be understandable, accessible and crucially clean-able!. Any system that transforms an object by an offset will encounter pops and hitches if the keys of the offset change, get moved or deleted. (Another reason to place them in there own area – attributes etc)
Charles Looker 