While I am pleased with the introduction of Dolby Pro-Logic II I could care less about other DSP modes. I currently have a number of additional modes on my Denon 3301 and I have never used any of them.
Take the resources (cost and space constraints, etc.) and apply them to other functions (although at the moment I can't think of anything you left out ).
I was always under the impression that the other DSP modes didn't impact price or other performance aspects of the receiver. So why not? Never know when you need to expand that mono source.