CapnZapp
Legend
I can't be sure of course, but I get the distinct impression the party gets their XP a bit too easy if monsters stop fighting while they still have HP left.Another way to look at this is asking yourself the question "How many creatures will actually fight to the death before surrendering". IMO it wouldn't be many, most would either attempt to flee or surrender when seriously wounded or when it is obvious they aren't going to win.
In 4E "real-life" considerations take a back seat (if that), so I wouldn't be surprised if GMs were to be recommended to cut back on the XP of any opponent that gave up before it dropped.
Remember that if you are a monster, even if your own life is forfeit, by sticking around, you can possible cause enough damage for your allies to carry the day (or, more commonly, cause the loss of enough healing surges so that the heroes have to take extended rests slightly more often than they would have had to otherwise)...
It's slightly different at "wrap-up" time. When the DM cuts short the battle when he sees there can be only one outcome to conserve dramatic tension.
But during the height of battle, when no outcome can be clearly seen, if monsters give up at single-digit HP (or worse, when they become bloodied) that should probably mean a significant reduction in XP.
Regards,
Zapp