I've been using Pine for many years (more than ten years actually), and it always been rock-solid and stable.
So why the switch?
Well, two things
- Pine is not true free software, as defined by e.g. the GNU General Public License. It is free to use, but modified version cannot be distributed.
- Pine has no real support for threading. And there are also a couple of other small technical things, that annoys me a bit.
Mutt seems much more hardcore, and not for the beginner. But then, I'm no novice...