I must say, my personal experience has been very different.
The game I run is at 9th level right now, and the party ACs are:
24 (cleric)
23+ (warden)
20 (sorcerer)
19 (wizard)
The warden's AC is often much higher due to second winding and other tricks.
I play in a game at 13th level, and the party ACs are:
30+(swordmage)
29(fighter)
27(druid)
25(warlock)
23(bard)
The swordmage often has an AC of 32-34 due to magic item and daily utility powers.
I play in another game, oddly enough also at 13th level:
28(paladin)
26(warlord)
26(ranger)
24(wizard)
So the point is, in one game, the spread is 5+, and in one, the spread is 7+. Only in the third game is the spread within the 4 area that most people seem to think is the sweet spot.
However, we've never noticed a problem with the defense spread causing issues in any of the games. If the bard is getting attacked in melee, the swordmage and fighter are already not doing their jobs. If he's being attacked ranged vs. AC, he is usually able to find cover, concealment, or another way to stop being the target of attention.
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gnfnrf