I find that style of max damage plus roll is a large power boost to rogues and paladins, both of which add dice to the critical.
5d6 Sneak Attack averages 17-18 points of damage. The same from a crit adds 47-48 points to the weapon damage. Even worse for a paladin who can decided what sized smite to add after seeing it's a crit.
How is this a much larger power boost? You take all the dice your going to roll for a crit then grab one die(dice) of the weapon type from your crit pool and set it down on its highest number then roll the rest. That isn't going to be that much of an increase. For a rogue that's a d8(or d4 or d6), for the paladin it could be 2d6(or d12 or d8, etc).