I'm finding it increasingly more uncommon that I have had the same agility instructor from the beginning (for the most part). I don't have any issues about training with her, she's respectful, so why wouldn't I stick with her? A few of my friends seem to go from instructor to instructor and never sticking with one.
I can't even imagine all of the reasons why, but I'm curious. Why do people end up switching?
The only reason I could foresee is a change of equipment, or classes. I would love to train with someone who only does NADAC as I don't get enough time to work contacts in general, working contacts on rubber or other non-slatted material happens even less. Class times could be an issue too.
I've never had an obedience instructor, so I can't comment on that front.