Playing a player in a position he hasn't already learned during ST will significantly increase his gains in experience at that position already (compared to playing him there during the regular season). That happens naturally, without any additional input from the player.
And positional ratings do currently atrophy; a player who doesn't play in a position for a while will lose experience at that role, although I don't think they 'forget' how to play there completely. Possibly not quickly enough, but it does happen.
Take this guy for instance, not the best example but he works:
http://i.imgur.com/Tt7SmOH.png
Started off as a left fielder, I moved him to first base ages ago (the first year he lead the league in PA was his first year at first base) because he can barely move, and now he wouldn't be able to find left field on a map.
I don't tend to see *too* many "super-utility" guys, with 60+ in more than two positions (unless it makes sense, like a guy with all-around fielding ratings being competent anywhere in the outfield). The game seems to get it about right IMO, where if a guy has more than a couple of positions, he's not very good at any of them. The more positions he has, less worse he is in any particular one. That type of 'jack of all bases' is pretty common in real life. What isn't so common is the Sean Rodriguez type, where they're legitimately good at fielding a large variety of positions. I don't see too many of them in the game though, to be honest. Most guys have a primary and a backup, or three decent positions.
Sometimes I'll see odd combinations (SS/RF or LF/2B for instance). I've currently got a guy on my roster who's 55 3B, 55 CF, 30 SS. Which is a little odd I guess, but I can't say I've ever thought of it as an issue.