Snakes can pick up parasites if left to roam outside, and as none of them would be naturally parasitic to foreign species it could potentially cause major problems. Its best not to do too often.
Mine come outside with just thought if a proper mesh viv was used mite enrich there summer lives abit with abit of variety. They wouldn't be free roaming as such but be able to enjoy the natural heat and light. I cant see what parasites they could pick up anymore than being in a normal viv if the outside one is properly treated and moved in when not in use