Shadows_With made a good point about the dementors' intelligence. They can't seem to lean to speak our language and they seem to follow pure instinct. I'm sure the soul of a lonesome little boy would be quite delicious, but when the most powerful wizard in the world (DD) is protecting the boy, attacking him may not be the smartest thing to do.
The Dementors in PoA do not seem to comprehend this. They seem to be instinctual creatures who must do what they are told.
Perhaps when somebodys soul gets sucked out, the person eventually becomes a dementor. I think that if a Patronus really wanted to, it could kill a dementor. However, the priority of a patronus is to protect whomever summoned it, so I doubt the patronus would kill anything unless absoloutly neccisarry. It would rather drive the threat away than risk being destroyed itself.
Just my two cents.
.......Oh yeah, and in the movie of PoA, surely I'm not the only one who hated the fact that not only could dementors
fly around, but they were faster than a Nimbus 2000.
