My indoors is A LOT more stressful than my outdoors. Kids and their friends, large dogs, multiple cats, very loud music. etc. We have a busy household.
I have no special bunny building, garage or porch to use.
The rabbits need desensitization just to be able to handle it, otherwise it's just too darn stressful for them to be brought inside for medical care or playtime, and it's to hard on the family to have to "keep quiet" when we have another rabbit in the house.
(Mucky loves being in the middle of everything, she's the exception)
I agree that fewer calories would be spent in keeping warm though.
and the water never freezes inside.
Summer weather is pretty perfect for bunnies in my part of PA, so it's really only winter weather that would cause an issue.