Yay to the new rule! It will be sooooooooooooooooo cool to wear bike shorts in competition too!
The long sleeve leos aren't just uncomfortable when it's hot hot, you can get hot and sweaty in like temps. in the 70's too or upper 60's, it's the hot and sweaty issue and also it's just not comfortable, I think bike shorts rather than long tight pants are more comfortable too. Comfort in a sport is very important.
I like sleevless leos with shorts - seems like a lot of gymnasts do as that is what they choose to workout/practice in when they have a choice. Liukin, Memmel, and Bieger even did the Visa promo in a sleevless leo and shorts too.