If you look at the only sort of similar venue with a retractable roof - Skydome or whatever they're calling it now in Toronto - they pretty much only use it if its raining.
Toronto doesn't get quite as hot and humid as DC, but it gets bad (bout the same as Chicago or Detroit). Only thing that saves it is the breeeze off the lake but, depending on the time of year, that doesn't always come up until the game is about over.
I think for maximizing your home wins, a retractable roof is always better (Nats already seen some losses due to rain). However, I doubt the prospect of winning a few more games per season is gonna influence the people who decide on the stadium design much.