To me it’s a usability issue. I use 3ds Max and most things import as to expatiation but doing 1-1 from DS to UE4 with unpredictable results is a deal breaker. Part of our project design the ability to let players make their own player models in the future is on the table. Overall sure G8 is a better product, even with stuff that is not needed, but would require a lot of design changes to things already done.
Forgive the the production value but here is how we make use of the framework
So as you see no point in muscles or joint deformers. For that matter we are more interested at some point of importing game assets into DS so we can change body types. (for another time )
From the department of when pigs fly though the ideal would be total DS integration with UE4 but Epic would have to buy Daz3D for that to happen (not like they have not done so before 
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By the way we also use Motionbuilder so the bind pose does not matter as to characterize the model needs to face the Z direction and in the T-pose position. This includes quads as well. Ever see a cow in the t-pose?