I think it depends on what people are using their editor for whether they'll jump ship. I started out on vim, and I really loved it. Then I was typing up papers using the vim latex suite and syntax highlighting would cause my machine to choke every time I scrolled through text. I could have just turned off syntax highlighting, but I tried emacs and auctex, and it worked wonderfully. Since then, I found a bunch of other things that I prefer about it, and only really use vim for quick config file edits. Now that I've been playing with it for a while, chords are second nature, just like modal editing became for me with vim.
I imagine that nobody who's terribly accustomed to either editor will switch unless they've got a specific need their current editor doesn't meet.
I had a similar experience, really. Do I want to use emacs to edit system files and such? Not really, I'll continue to use vim. But for programming projects and document writing I like emacs better. To be honest I think the thing that started it was messing with Sublime Text; after using it for a few weeks I started tinkering with emacs again and never looked back. I will say that I never fully bought into vim in terms of movement keys and in depth shortcuts; I pretty much used cursor keys and the mouse and didn't do anything too advanced.
I actually started using Emacs at work when work became dangerously boring and I was looking for a distraction. Plus I had read a lot about Emacs in my Vim days and wanted to know what it was all about.
So I suppose I actually switched without a specific need except for curiosity and boredom. I'm glad I did though, since Emacs has improved my life quite a bit since then.
I imagine that nobody who's terribly accustomed to either editor will switch unless they've got a specific need their current editor doesn't meet.