Umbran said:
His colleages, being still active, are going to beat on you while you make sure their friend really is down and out. Cdg, or otherwise stopping to make sure he's dead, not only gives the PCs a "free round" of action against you, but it also gives them a darned good reason to focus on you, specifically. While there are specific situations that make this reasonable, as a general rule stopping in the middle of combat is not tactically sound. You don't deal with things that may become be a hazard while there are still things present that are currently a hazard.
There are two points to be made here. Firstly, healing magic outstrips capability to deal damage at high level. Secondly, anyone dead is effectively out of the battle period; whilst someone who is down can quite easily be healed again.
Let us assume you face two opponents, one down (on e.g. -1hp) and another uninjured (100hp). It is almost certainly strategically better to attack the downed one unless you and your allies can take out the unwounded one before healing magic is applied. If you focus on the unwounded one, assuming that more healing magic can be brought to bear than you can deal damage (almost certain at high levels), then the opposing cleric can negate any 'progress' you have made that round. Say you take the guy from 100hp to 50hp, and then the cleric heals the downed guy from -1 to 50. Now, you have to face attacks from two frontline fighters- or worse, one assault mage and one frontliner. In the next round, you might be able to take one of them from 0 to 50, but then the cleric heals him up and you are, again, subjected to two sets of attacks.
I suppose the argument could be made that it is worthwhile so long as the cleric is more dangerous than the combatant whom is down. However, this is rarely the case. Arcane casters and druids have access to more dangerous combat magic than the cleric; if the downed combatant is another cleric then it is far better to have one cleric get an extra action than two clerics run around healing everyone; tactically-optimised rogues can deal more damage than a cleric; frontliners generally deal more damage than clerics in the long run. In a closely contested combat, clerics on both sides are likely to be primarily fulfilling the role as healers- especially given that you are fighting an adventuring party (as 'monsters'); NPC clerics might be more aggressive with their actions.
Ultimately, it's not a question of whether a downed combatant 'might be' a hazard- he almost certainly *will* become a hazard. Party clerics almost always heal downed party members first before attending to wounded but standing ones, unless there are exceptional circumstances. Particularly at high levels, unless you can kill an active combatant, you're almost certainly better off taking out downed ones. As the levels increase, this becomes even more of a tactical must- once Mass Heal rolls around, killing downed opponents is a no-brainer.