Until they tweak the AI, standing starts could be a disaster. Each car may have different grip levels and acceleration. Lot's of tightly packed starts gives many passing opportunities for the AI. If the AI isn't up to snuff it could be collision city.
It's also a strain on the graphics engine to have more cars in a tighter pack. but I think the reason for rolling starts is more likely to not expose the AI to a lot of crashes. I read a couple reviews, which speculated the same thing, but it's still just a guess.
I prefer standing starts, but there are many real world series with rolling starts as well. It does keep the accident count to a minimum, which could be a real issue with the AI logic in offline. Online, that's not an issue. It may be that PD just prefers rolling starts.