I would also like to second that casters don't really have a low AC, except maybe the warlock, and he has other things to help him out.
Disregarding that for a moment though, in
4e the game is designed so that people don't only miss on a 1 and don't only hit on a 20 (there's some debate as to whether this still happens at the highest levels, but for most of the game this holds true).
That means a -2 to attacks is a -2 to attacks is a -2 to attacks. Even if my wizard can get hit on a roll of a 5, if the defenders mark makes it so the creature needs a 7...that's still 10% less chance on the die roll that he will hit me.
And of course, defenders tend to be pretty good against archers. If a fighter marks an archer, he's done for. Shifting provokes an attack, attacking the fighter provokes an OA, and attacking someone else without shifting provokes 2 attacks!!
A paladin can force autodamage on an archer and a -2 to attack rolls. With with charging, its not hard for defenders to get into the back line.