I know some of the OS drive has to do with asthetics. With frame tubing generally being larger in dia now, the OS bars tend to look more balanced on the bike. I have heard this from a few Product Manages and a Handlebar maker. I tend to agree.

I think OS will pretty much become the standard within the next few model years.

I think the pros would be a weight savings and a wider selection with OS bars. I don't really see any significant cons to loosing the quill stem and 26.0 or 26.4 bar.