I agree about the defense part. Simmons has great defensive energy, and he has the athleticism to keep up with PGs, but his technique and discipline are not up to snuff yet. He's not an upgrade to Parker defensively.
I think you're selling Simmons short on offense, though. He is a pass-first guy, and he has shown that this season. If he gets into the lane and can create an good look from someone, he'll do it readily. The only thing that might be holding him back is chemistry -- he may not have enough experience with guys to know where they're going to be at any given time. That makes finding teammates much harder.