This is always an interesting debate. Yes, that's always been the "conventional wisdom"...but break it down for me (without snarky insults, please).
Whether you hold the ball for 3 minutes or 8...you still only get one opp to score. There are no points for riding time. The opposition gets it following your possession either way. 1 for you, 1 for them. There is no "number of possessions" advantage.
The only rationale I can see is that you assume the defense is always at a disadvantage (offense knows the play, defense has to react) so a defense will fatigue quicker than an offense...therefore you'd rather have your offense take the majority of the snaps so the oppositions D reaches that point of fatigue before yours does.