Right, but how would you award XP and treasure for this? Sorry if I wasn't clear in my original post. Obviously I could roll up some dudes and have them fight my players, but I wouldn't know how much XP a 10th-level fighter is worth; that's what gives me trouble. Unless I'm totally missing a set of rules that addresses this, in which case I feel dumb, I wouldn't know how much of a challenge an 8th-level party of evil adventurers would be to a 10th-level party of PCs (for example). That would make it tough to balance the encounter and award treasure and XP.
As to the issue of why should all wizards feel the same, of course they shouldn't. But it always seems weird to me in 4e that a 10th-level PC wizard has dozens of powers to choose from, whereas a 10th-level monster wizard only has a couple.