I think all the players in my game have used retraining at one time or another. We've also had one PC rebuilt from the ground up, when the hybrid rules came out (went from ranger multi-classed cleric to hybrid ranger-cleric).
One player in particular uses retraining extensively, to tweak his PC from level to level as his conception of what he wants his PC to do/be changes, or if it turns out that an option didn't work as well as he hoped it would.
It doesn't seem to do the game any harm. It certainly increases the number and variety of mechanical elements that see play. I can see how this helps WotC's bottom line (in selling those mechanical elements) but it also contributes in a modest way to fun at the table too - part of the attraction of a mechanics-heavy game, after all, is seeing the mechanics in use.
There's no denying that this retraining player is also the player who takes the most interest in mechanical minutiae in the game. He is the same player who, playing a quickdraw samurai in Rolemaster, had Excel graphs to determine his optimal distribution of combat bonuses between initiative, to hit, to crit, and defence for various numbers and varieties of foes and situations. But it is also this same player who circulated among the group the most recent and up-to-date version of the players NPC/location/god/stroyline tracking lists. So there's no evidence at all that his interest in the mechanical side of his character is at odds with his participation at the table.
Ultimately, if you want to play a game in which players don't pay regard to the mechanics, you need to play a mechanics-free game. D&D is not that game, and has not been for some time now (I'd say at least since the Survival Guides were published for AD&D).