I don't think he necessarily makes us better either. He scores a lot but at the sacrifice of other players and defense. He worked in OKC because they were a high octane offensive team and he just added more to that dimension they had. They could outscore teams with Durant and Westbrook and himself with guys like Ibaka inside. They still were not able to win a championship, even with all that offense and athletic big men who could roam around and disrupt shots.
Listening to Harden talk about how he is great at his press conference, I question that somewhat. He may be great on offense but he does it in a way where he puts all his energy into scoring points and does very little else. Is that the definition of a great player?
To answer your question I don't know how many wins he gives us. He could make us better or worse. I think we want to be able to utilize Al Horford who we just signed. He'd add another offensive dimension with Isaiah and Horford, but our defense would suffer. We have some good roleplayers there. Depends on what we give up. If we are giving up marcus and bradley and amir and guys like that we would be a pretty bad defensive team.
Harden's coaches seem to think he'd play defense if other people scored, and the onus wasn't on him to do that so much. But that's also just his game, at least the last few years in Houston.