When developing a game for the most popular online phone game store will net you $250,000 in two months, as an independent developer:
http://toucharcade.com/2008/09/19/trism-developer-makes-250000-in-2-months/
The logout kickstarts the windowing system again, they could rewrite it to make it switch on the fly, but that would probably break something else, and not something you would expect in a 10.5.X release.
Of course, they could write their own MacBook Pro specific windowing / graphics code, and just have that as a 'temporary' solution that supports live switching until 10.6 (where the entire graphics engine is having a major overhaul), but that defeats the whole "write once, use many" design philosophy of Apple's development cycle.
So Apple adopted a slick, bleeding edge tech, but instead of holding back for them to be able to make it work 'as it should' they introduced what could be considered the best practical work around that would involve the least amount of damage. I would hate to have to test any code that uses the graphics engine specifically on the newest macbook pro, because Apple decided to break their own development procedures to provide 'live' switching in a service pack. I'd rather just wait for the code to be in place globally, and have more incentive than "it sometimes breaks on macbook pros" to rework my code. Like being able to utilize a lot of the new low level stuff coming in 10.6.
The one day you'd sell your soul for something, souls are a glut.