As a fan of the big-market Red Sox, I always thought it overrated to keep a player in the minors for a month or two just to keep one more year of control. If he proves he's good enough for Boston, then pay him market value to stay as he approaches free agency. Better yet, if he's worth a longterm investment, lock him up asap. That seems to be the big plan these days, anyway.
If he can help the club and is one of the best 26, sending him down is unfair to both the player, and the fans, who always want to watch and root for the best possible team.