It does happen - Monty Panesar played for Bedfordshire in 1998-99; but it usually works the other way of course. The MCs are full of ex-county players, some of whom had a long career and some who just had a few matches and didn't kept kept on.
Its interesting that not many players seem to progress through from Minor Counties, i guess if they are good enough they get snaffled by a County team straight away.
I'd have thought that Minor Counties cricket would provide useful experience for players.
Yeah, they're usually spotted by county 'scouts' when they're 17 or 18, and most young players would rather play for a first-class county if given the choice. The only mature player I can think of going to a f-c county was a guy called Nick Folland who moved from Devon to Somerset for a couple of seasons in his late 20s. Like a lot of MC players, he didn't want to play full-time. So he ended up returning to Devon.