Thanks everyone.
I appreciate the input, but...
1) Is the rule really written like this? There really is no mechanical cost to choosing the Knocking them Unconscious option?
2) What would be a reasonable cost? You're essentially fighting with your blunt side rather than your sharp side of your sword. Would a to-hit penalty be appropriate? Or some damage penalty? (The cost should be enough to encourage players to use this option only when it really matters to them, but not so big so they feel the option isn't there at all)
Hasn't this been discussed already? I sort of expected links to previous threads on the topic
It has come up before, although they've probably dropped off the front page.
In general though, with the PCs, going unconcious isn't a case where you either die or you live, you could concievably be "dying" for the rest of the fight. If no one stabilizes you (and you don't pop up with a 20 for your save) you would probably die eventually.
For the NPCs, it's basically a matter of whether the PCs try to save them or not. It's another case of simplification ... the PCs either decide that at some point in the fight, they did enough non lethal damage to knock the person out instead of kill them (if you want to look at it that way). Or they stabilized the person before their 3rd failed death save. Instead of making things complicated, they just said "the PCs can choose to have the opponent's survive being knocked out". Very rarely does a monster die before it hits the ground, most "death" in D&D is basically passing out and dying of shock to some extent, rarely, in 4e, does someone go to negative bloodied unless they are under som sort of ongoing damage or get caught in an aura/blast/burst that finishes them off.
Instead of making it harder in combat to take someone alive, have the choice between letting the opponent's live or die by an out of combat issue. The players see the benefit of questioning the bad guys after the fight ... if they have all upside and no downside, they'll keep the badguys alive. It is probably best if either option is viable, so it's not a case of railroading the PCs, or punishing a player that might be playing a character that wants to redeem evil people, or bring them in alive, etc. In general, if they keep the bad guys alive longer than just for interogation, have the bad guys become a drain on resources and a nuisance, but perhaps with an upside (the town has posted a reward dead or alive, and will pay more if they bring them in alive). If they get the information and let them go, they may end up with enemies willing to surrender to them and offer up information in the future, or they may end up with recurring foes and perhaps better prepared enemies in the future that know how they fight. If they kill their prisoners after getting the information they want, it may get out and cause people to be less trusting of the PCs, less willing to talk, etc ...