Maybe they will reduce the HP for monsters and such.

Nerfed Crits would mean enemies stick around longer with high HP, and frankly that's another reason I think the rule is bad. Most players I know do not love long and drawn out fights. Fighting a monster with 200 HP is cool when you can do 20 damage at a time or more with crit doubling your potential to 40 or 50. Fighting a 200 HP monster when you can do maybe 10 to 12 normally and 20 at most with a CRIT is not as much fun... Unless maybe the monster's AC sucks and it's easy for everyone to hit.

But as it stands, only weapon damage being rolled again means the level awesome rogue with like 6d6 sneak attack and 1d8 rapier with +2d6 fire damage gets a CRIT and does 6d6 +2d8 + maybe 4d6 if they count the magic fire damage too. Quite underwhelming. On average, maybe 40 points of damage for a crit, a whopping 10 points on average more than a normal hit average which would be closer to 30. Wha wha!

Now double all damage dice and you feel like the crit means something. 12d6+2d8+4d6. An average of close to 70 damage. Heck yeah! Now we're talking. That's a Bard killing Smaug with a black arrow kinda critical hit especially if the target is vulnerable.