Swivels are for things that spin. Hopefully, the kite in flight is not an object that spins.... Therefore it is not a good idea to use a swivel on the end of a flying line. The reason being that it is impossible to determine the condition of a swivel before flight, and it is a 'mission critical' item. If it fails, it generally means the loss of the kite.
Just a quick reminder about swivels: Swivels, even the good Sampo ball bearing swivels, are designed to be used in water, with water flowing thru them to cool and lubricate them. In the air, holding on to a turning piece of laundry, there is nothing to cool or lubricate the swivel. They should be oiled at each use, either before, during, or after.
Another good trick is to install swivels reversed (pointy end away from the wind) whenever possible. This reduces the amount of sand and dirt that gets ingested by the swivel.