Korver's defense is whatever it is. He's not jason kapono, nor is he david lee. He's a perimeter defender. People are goign to beat you off the dribble on the perimeter any day of the week.
here's the thing about starting korver. He's going to get his minutes. Whether it be off the bench or starting. The argument isn't whether korver should be playing 25 minutes or 30 minutes. The argument is whether he should start or not. Korver is probably going to get 25-30 minutes a night. Brewer should absorb the rest. If korver starts, he's going to see playing time with rose and boozer on the offensive end. This will help him shoot. We'll also see him with joahkim noah, whose a really really good help defender. Also he's the best pick and role defender in the league.
On the bench you put him with cj. You force cj to look for him off screens. It throws cj off. Starting bogans throws him off for the simple fact that he's not good enough to be a starter. Also Brewer doesn't really need plays called for him like korver does. He hustles. It makes more sense to let the starters create for korver, and let cj do his thing off the bench. Also asik and taj will be his help defenders. Taj is a better defender than boozer, but asik isn't as good as noah.
The caliber of nba 2 guards aren't such that you're going to get killed by korver on a nightly basis. Besides wade, kobe, evans, and ellis there aren't too many 2 guards that kill you with athleticism. Guards like JAckson and kevin martin's games can be neutralized with good positional defense. Korver isn't a bad defender, he's just slow. He can play good positional defense against a lot of these guards. He'll get killed against the athletic guards, but sub in brewer for those guys. But the majority of the league is guys like affallo, brandon rush, whoever the knicks are starting, parker/gibson, jamal crawford, and raja bell/cj miles. You're not gonna get killed by starting korver against those guys.
plus as i've already said, if korver is getting killed, put brewer in. Starting a guy doesn't mean you have to play him.