The game has changed in that players are getting more rest days and spending more time on the DL. Thus, depth has become more important. However, I agree that there is nothing virtuous about putting together a roster of guys that play all different positions and havin not set roles. Platoons are great when it's Champ Summers and John Wockenfuss at designated hitter on a consistent basis, not so great when you've got a different second baseman and third baseman every day and none of them are elite fielders or hitting enough to be starters.
The Athletics had a revolving door at second base in the 70s because they didn't have anyone who could play every day, not because they wanted it that way. It shouldn't be any different today.