I don't understand why you assume the following things
1. There is a average damage per encounter that is supposed to be there.
2. Solo monsters are alone in combat.
3. The number of monsters that "should" be alive per round in a normal encounter.
4. How long each combat is "supposed" to take.
5. That solos should do close to as much damage as a normal encounter on their own.
6. That solos (lvl 11 solo is what we are discussing) automatically lose two rounds to random debuffs.
7. Your last statement just seems to assume that his "utility" doesn't make up for the extra damage without giving any reason why.
1. You need a certain amount of damage per fight or there is no actual threat. There may be perceived threat, but that doesn't work on a fair number of people (although perceived without actual threat _does_ work on a fair number of people too...)
2. I don't think anyone is assuming that, though by the RAW it should be a possible option.
3. Those were off the cuff numbers, but the important difference between a solo and a regular group of 5 creatures is that you steadily decrease the damage output of the 5 by killing them off. You can't just make a solo have 5x the damage of a normal creature, because it will retain that damage for 100% of the fight, instead of losing it steadily.
4. Combat lengths vary drastically from group to group - for example one group I know with 4 strikers in it tends to average about 3 rounds at heroic tier, while I know at least one poster on these forums who averages over 10.
5. If solos don't do as much damage as a normal encounter, then they're not threatening. There are many who'd actually suspect they should deal more than a normal encounter, but the motive there doesn't seem very mathematically pure.
6. It might lose any number of rounds, but between massive attack penalties, daze, mark, immobilize, knockout, what have you... it's very likely to waste more effective rounds than a group of 5.
7. What 'utility' are we discussing? I assumed that it would survive longer than a normal group of 5 creatures by 3 rounds / 50%, via defensive abilities, heightened defenses, flight, fear, etc...
It's very important to realize that a fight that goes for a long time against a creature that is hard to hurt, but that in turn can't appreciably hurt the group is almost invariably boring, at least to those who can realize it.
I mean 'So, guys, I know it's going to take 10 more rounds to kill this guy, but at the rate he's damaging us, he'll drop the paladin in the 50th round, so... whee' isn't exactly optimal play
I threw a young black dragon at my pcs and they still think it was their most difficult fight. It wasn't about the damage, it was about getting past its wierd defenses.
Black dragons have immense defenses... which leads to long fights with lots of at-wills, but not necessarily any true fear or excitement. It _can_ have those, sure, but it can also have a group that no one has gone bloodied in, no one expects to, too.
I actually really don't like the design of the black dragon, since it's mostly a denial fight.