I like that trade for GSW.
Golden State already had two elite reserve combo guards in Lou Williams and Reggie Jackson so Ben Gordon value was minimized there. DeRozan, however, gives them a very different look at 6-6/6-7 220lbs. A big two guard with great athleticism. In terms of creating matchups, DeRozan is the more valuable piece to GSW.
I don't think I'd start DeRozan though. I think Brandon Rush as a defensive stopper is a better fit in the starting unit but I do like the options DeRozan brings off the bench as a backup SG.
I would share this enthusiasm but it's all smoke and mirrors. You're right that Demar's better off coming off the bench because he'd be a poor fit in the starting unit, but where is he going to get backup SG minutes from? You already mentioned Williams and Jackson, and they'll be soaking up most of the bench guard minutes. That leaves Rush alternating between starting SG and backup SF, which would mean Derozan would still have to see the court with Gay. Or, Derozan could instead be the backup SF, but he hasn't posted great numbers whatsoever playing that position, and would likely see very limited playing time anyway with Gay soaking up at least 35 minutes.
Who cares? Seriously. Who cares about DeRozan's minutes?
He is the 9th-10th man. That's it.
Not some integral part of the team who you have to find minutes for.
If there are minutes available where he can help you, he'll get them. If there aren't, no worries because you are not forced into relying on him for playing time to begin with. He is an end of the bench player. You don't worry about minutes with players like this.
DeRozan is just someone who gives you an extra option. A different option off the bench than you had before. A valuable option to have. That is a good thing. It is that simple.
Golden State improved themselves here.