Beyond the character development argument (which is totally valid on its own but need further explanation I am to lazy to give), harems usually ruin most games, as it kills the necessity of the player to get deeply affected by a character as he can have them all.
Oversimplifying: a choice is as good as what you leave behind. If you can have all, you are not choosing anything.
This is particularly true for pure love stories, as in those stories the only source of drama is getting the girl. Once you get the girl, there is not conflict left to be resolved. Now a harem kills all conflict possible as the main character can have it all.
I don't think this is a pure love story, but I hope for many reasons this is not a simple harem. Multiple character relationship, IF they make sense, are ok, though.
EDIT:
Ohh good you explained it way better than me.. I am feeling too lazy now so my explanation kind of sucked