Very true, when I played the game I got the distinct impression Shinji was running a brothel in his spare time
The fact that he is so different in the game compared to the anime still derrives on how much you actually see him. If I remember of the game, he visited Haruka 3 times in those 3 years (now I could be way off on that count, been a while), however they never really shed too much light into how he spent his time elsewhere. You only see him at key points in the game, such as when he and Mitsuki did their little thing...something that defines his role as supportive and not so much a direct impact, albeit he was a strong supportive role
I swear, I'm going to have to switch to my XPj and play it again and hope like hell my translation skills improve a lot better than what they currently are lol...
I'm also figuring the change to the anime was so that they wouldn't have to increase Shinji's screen time, thus actually taking away from the issues that Takayuki and Mitsuki faced. Even as much as Akane was in the game, I think she got more explained in the way they showed her on the anime...there was a different feel to her in the anime than there was in the game, I just can't put my finger on it though.