More games = more at bats/innings pitched = more opportunity for development.
I've had several guys get their 5th development mark during the minor league postseason (though certainly some of that improvement would have occured at the tail end of the season and would have appeared in the next spring training line, or promotion line for the guys I promote at season's end.
I don't know that its a huge boost, maybe a skill point here and there at a player's peak, and of course there's still the risk of injury, but I always prefer my minor leaguers to play in the post season than not play. I don't load my minors w/ AAAA players to garuntee it, but at worst I make sure each level has proper depth at each position to help with injury or players wearing down. Often times, that enough to get you a look at the playoffs. Considering most minor league teams have 1-3 legit ML prospects at best, its more work than some coaches are willing to do during the FA period to ensure their minors are propperly stocked, but I think its worth the effort.