And still your fight would complain even more, if your thief had DPR of 5, but does 30 Damage in a round, when he can kill a bloodied foe.
Actually this is the essential assassins role. Strike where he can take out a foe. Maybe one at the beginning of the combat and than going where he can reliably take down a foe.
DPR is no measurement at all if comparing different kind of strikers. A single nova, which the thief is surprisingly bad at without the right feats, can do more harm to the enemy as 2 points of damage per round...
DPR in a theoretical hack-beeing hacked-hack-being hacked is no measurement of efficiency.
You do have a point, but I think it's worth some elaboration. I would add the following considerations:
1. Many creatures have significant advantages once bloodied -- regeneration, recharging of powers, etc. If you can take them straight from unbloodied to dead in a single attack, bypassing all of those advantages, that is a big win.
2. Many control powers have an extended duration. As such, it is often better to focus on killing the creatures that are not currently being controlled, while you mostly ignore the controlled creatures and let them make their attacks at -6 to hit, or half damage because weakened, etc. You then quickly finish them off when their allies are all dead.
3. Many combat encounters feature a dependency between creatures, for example a BBEG with minions who might surrender, run or die when the BBEG falls. Taking the BBEG down quickly could be critical.
However, there is a downside to a focus on Nova potential -- it requires a knowledge of the enemy's capabilities, current status, organization, etc., or at least a good instinctive hunch. For example, expending a Nova power to take down an illusionary copy of the BBEG might turn out to be a poor choice. Piling on extra damage to knock the bloodied foe from 50 HP down below 0 doesn't work so well when it turns out that he regenerated and has 60 HP after all, or only had 10 for some other reason.
I think UngeheuerLich may be going too far in dismissing DPR outright, but it's definitely true that DPR itself is only one measure of effectiveness.