aboyd
Explorer
Yep. 5th level wizards, 6th level sorcerers -- at that point, they gain 3rd level spells, and that's a deal-breaker for lower-level PCs. If you have a 5th level wizard NPC and he's got his first Fireball spell, he's going to bring all the PCs down by an average of 17 hit points in the first round. So even if there are 6 3rd-level PCs, odds are good that in the first round, only 4 are still standing, and it'll be pretty desperate for the remaining 4 to endure subsequent rounds of battle. All the PCs will be close to dead after just round 1.
Or, another angle: maybe that wizard doesn't have Fireball, but does have another 3rd level spell, Fly. At this point, he's dropping rocks onto the PCs heads and they'll have very limited ability to get up near him and enter melee. While the wizard is beatable, he renders the melee types obsolete or much less effective for the course of the battle, and thus it's much more difficult to win.
Or, another angle: maybe that wizard doesn't have Fireball, but does have another 3rd level spell, Fly. At this point, he's dropping rocks onto the PCs heads and they'll have very limited ability to get up near him and enter melee. While the wizard is beatable, he renders the melee types obsolete or much less effective for the course of the battle, and thus it's much more difficult to win.