Playing it smart IC. Unless there is a compelling reason to keep an enemy alive, you should kill them. Letting them live means having said enemy to deal with again in the future.
This. Unlike Superhero books, most people tend to feel that letting the powerful thing that's been trying to kill them for some time get away will come back to bite them in the ass. Even if the GM makes a new villain, the CHARACTERS don't know this will happen. If they did, they'd see it as having two threats to deal with instead of one and still kill to opponent.
Concessions are awesome. So is a taken out result that doesn't involve death. They are great things. However I think there are some key points:
If you are fighting to the death (which often you won't be depending on the scenario), it's important to Concede while you still have a chance of kicking ass. You've taken your licks. You've got some consequences. But so have your opponents. There's a very real chance that you could kill them if the fight goes on, but also a chance that you'll be killed as well. You concede to live and fight another day and they do too. Staying to fight is an option, but not a risk you're willing to take.
I think this should be done across the board (for PCs too) so that PCs aren't asking for concessions after fighting to the very last against monsters that want to kill them. If you're escaping, your opponents have to have a reason to not just run you down.
An exception to this is if you have an escape plan set up. You drop a building on someone, hop into the Nevernever and lose them, Joker Dillema, etc. Even then though, you're not at your last. You still have SOMETHING you can do.
Taken out not involving death is a bit different. That's set by your opponents. To get this, you should justify why they DON'T want to kill you (capture for ritual, need information, can sell to higher bidder on ebay, boss wants revenge, etc).
Basically, if your opponent is handing you your ass without taking much in return, has no reason to want you to live, and you don't have an escape plan...you're probably dead.
Most people/opponents/characters should probably be wary right around the time they start taking Moderate Consequences that they should go (hell, as a GM I might even compel the consequence as a "Hey, you can keep fighting but if you don't get that fixed you might die a few hours afterward...") to suggest a character consider concession before they've got nothing left.
With players, winning the conflict is important. If you want your enemy to get away, they can. The characters don't have to be happy about it, but the PLAYERS have to be satisfied with the encounter (they accomplished their goal, had a reason to let the baddie escape, or wanted the fight over too).
Also, Wordmaker makes some excellent points on (I'd address further but it was posted while I was writing this).
If your PCs kill something that will leave a body, there are consequences that can follow. Use them. Even if they dump the body, they might still be seen or leave evidence behind. People might notice that someone is missing. If it's not human you've got the Accords to fall back on.