Why Butyl vs Caulk under roof vent flange?

Either will work for a roof vent but I would use the butyl tape. We did some testing of various ways of sealing airframes were I worked and all of our Blackhawk helicopters were sealed with butyl caulking between riveted skins. We also used this caulking to seal composite air frames. It is more flexible does not dry out or shrink like other materials. There is a lot of different loads put on these airframes in flight and things move, not on like a MH traveling on bumpy roads. I have seen 20 year old airframes come in for overhaul and repair and the butyl seams are still good in spite of rivets and Al skins being corroded. MH's do twist, and we have all heard of wind shields cracking when improperly leveled on some MH's, so that is why I think it is a good thing to use a flexible sealant like the Butyl tape.