The objection of the anti-button mashers isn't that Gamer A can make Adrian Peterson or Reggie Bush run faster than they're top rated speed, it's that the
only people who can make these players play up to their ratings are the ones with highly developed button mashing skills.
The problem becomes even more pronounced on defense. In any sort of footrace situation (with all other things being equal) the player on defense is at a huge disadvantage, especially if said defender is at any sort of an angle relative to the ball carrier (i.e. he isn't either directly behind or alongside). If that's the case, it's game over. All too often you'll find that it's impossible to switch from tapping the speed burst to hitting the dive button quickly enough to make the play.
Keep the button mashing in Tecmo where it belongs.