Ive always had this argument with some people. Yes, Scenarist does let you construct a disc with no abstraction layer which in turn gives faster response time from menu to menu, VTS to VTS ect. But I say WHO CARES! I used Scenarist for several years and for really complicated projects I would use it. But for Low to mid level work DVDSP is great.
Never really had any major issues with it - never had people complain they couldn't play a disc because of DVDSP's abstraction layer
In the end the user only really cares about menus and functionality. Take great menu design and construct a DVD using both systems - people will not be able to tell a difference.