The reasoning, I suppose, was that it just added more dice-rolling- and that crits are rare enough and fun enough that you should just let people have them. I just never saw the sense in making people roll again to see if they hit if the roll already (Only a natural 20 is an auto-hit, of course.) says they did.
So, the questions for the hive-mind: Do you/Did you use the threat rules? If so, did they slow things down? Or have your 3.5/Pathfinder games been just fine without them?
This is all just out idle curiosity, by the way- they way I do it has been working for my game, so I'll stick with it.