Yes in those situations such a clause makes sense. The serial format is unforgiving of off season changes. I'd have to check, but I think they might have put Cuoco in a wig at the start of the season to match her hair from the end of the previous season. Her hair seemed longer right after the season premier.I can't speak for Felicity or even BBT, but quite a few shows run at a different timeframe than they air. Take the Walking Dead for example, a drastic haircut, change of facial hair, or noticeable tattoo for instance, could really screw things up or require a ton of cash to cover up with makeup/wigs/etc. And it would be just downright annoying (and expensive) if they changed it on a whim even during the season.
Consistency can be really important at times.