Crits from the monsters interfere with cooperative story telling. If a player overcomes great adversity to achieve their story arc, the last thing they generally want is to die to a few lucky/unlucky rolls. This is especially true when you consider that monsters often outnumber the PCs, allowing them not only more actions but more crit chances. This often leads to DMs having to fudge damage or hit results.
To me this means that the default rules should be more on the controlled side, knowing that experienced players will use whatever rule they prefer at that point and for their group.