Switching between systems.
I, personally, haven't had that experience yet, but I was so surprised to learn that one of my favourites, a well known photographer Scott Bourne, a Canon user, decided to make a move to Nikon.
He justifies his decision in this blog thread.
Takes some time to sell stuff, buy and get accustomed to the new, for sure. But when you really know what you expect from the gear, it's never too late to change.
After reading the blog I carefully examined my own experiences with my Pentax system and (gladly) ended up with conclusion - I don't need to move :)
Let me quote something from Scott: "It's just a camera, not religion".