While I agree that optimal processing will be unlikely with unmatched speakers, I suspect that in most cases, it will still sound better than what most people would be able to come up with on their own with a dedicated DSP.
Also, I suspect that even within a family of speakers, the cross-over setup is optimized for only one of the family tops. I seriously doubt that the cross over settings would ever be the same for a 10" top as it is for a 15" top (as an example).
I guess what I am saying is that the simplicity factor likely trumps the "perfect processing settings" factor for the lion's share of people using these products.
Personally, I had to learn these things when I had a passive system. Now it is kinda nice just to plug XLR cables in and turn everything on