Personally, I see no reason for it at all unless you have a very small dog and you have a lot of hawks or similar birds around or you live in an appartment on one of the upper floors and don't have easy access to grass. If you live in a regular house, have a yard and are not at a huge risk of having your dog taken by a bird I think that no matter what the weather or the size of your dog, it should always be taken outside.