Finishing off isn't the problem. The problem is that a particularly nasty critical can kill a character outright.
The redcap I used as an example is CR2. The book dictates that he power attacks for a 2d4+7. With a x4 crit, any L2 character will be bisected. The obvious solution is not to use him - he is fairly exotic anyway. But orcs with axes can be found in every mountains.
A fighter that takes a x3 crit at the beginning of a fight might go down or stay on his feet. The same crit in the midle of a fight might take him straight to -10
The topic was actually inspired by my GMing failure from last week. The party (non-optimizers) was chasing another adventuring party who had stolen their dungeon key. Rather then chase the enemy into the dungeon, they opted to stay outside and prepare an ambush in the ruins at the exit. They spread out to avoid area effects by the enemy casters - which proved unnessessery as the team sorcerer's stinking cloud worked like a charm. They managed to cut the enemy down with little damage; the only one really injured was the sorcerer's animal companion snake, who blocked the way to force the enemy invisible rogue to reveal and ate a sneak attack. Just then the enemy fighter, the last remaining member of his team, shook off the stinking cloud effect.
While preparing the encounter, I worried about many things the enemy might do to the players (they actually got to fight together in a previous encounter, to give them the feel of the other team's best tricks, especially the bard's confusion) As it turned out, I disregared two key factors. One- at level 7, low tier characters can still be a very credible threat and two - a big guy with an axe isn't useful for many things but actually very good at one thing- hitting people with an axe. As the result, the enemy half orc fighter, whose main role in the adventure was to drop a belt of strength as treasure proceded to use it to one-shot the two of the party close combat characters over two rounds, forcing the rest to save themselves by jumping out of the window and pretty much running around the courtyard until the halforc succumbed to multiple acid arrows. Fortunately they managed to end the fight before their friends bled to death.
I'l remember that lesson.