A cleric can give that bonus to one ally, if she hits each round, if she's willing to use the same at-will over and over (which means, short of an action point, she isn't using any other standard-action powers). What about the other 4 PCs, including the cleric herself, who aren't getting any bonus from the cleric at all for that round? What about the rounds when the cleric misses?
If the cleric has a +8 Strength bonus, and hits 50% of the time, then the bonus that the average party member can expect to see from Righteous Brand is +0.8. (+8, divided among 5 party members - the cleric counts too! - times 50%.) And that's a power bonus, so it won't stack with most other cleric abilities.
Then consider the party might not even have a leader. Or the leader might not be loaded down with powers that grant attack bonuses.
I think +2 is an entirely reasonable "average leader bonus."
I am talking about Astral Sea (or something like that) it targets WILL not AC.
There is also at least 1 other power (I am sure there are more) that is an encounter power that gives the entire party a bonus (not sure how much). You can also assume (which I think you did) that everyone can get combat advantage every round. (There are powers for that, as well as situational)
If the party does not have a leader, then all your assumptions are thrown out the window as well. What if they don't have a power that targets AC, what if, what if. We can only talk baout the assumptions that a party has 1 person that fills each role, and they are passable (that doesn't mean sub par and it doesn't mean optimized) at that role.
There are other things that are not taken into account:
defenses other than the highest one
some monsters are just tougher than others (and the PCs!)
What about powers that are (score mod) +2?
I don't know about you, but my party, once we start figuring out that there is a low defense, we tend to attack that defense more than the high defense score - by ALOT.
IMO, powers, tactics, and teamwork are too varied to make a blanket statements about bonus and hit percentages. There are too many situational things and too many choices to make in combat and character design to come to any real conclusion, unless you make a big assumption that the party fights the same way. The party would also have to use the same tactics, powers and spells, and have not situational conditions in each combat.
Again, I do not think most groups want to be able to hit the Stirge at 1st level with a 12 and Orcus at 30th level with a 12.
Epic monsters should be tougher and require better tactics, choices, and putting themselves in a position to succeed even if the PCs are Epic themselves. (again in IMO)