Kare Karo is shoujo and so uses shoujo mechanics.
And that is part of my point, harem is simply a mechanism ... it can provide comedy or drama, there is nothing wrong with it as long is used correctly.
Shoujo mechanics do not work very well (Bokura ga Ita is a example of a VERY BORING animu) for a male demographics because we REALLY do not care about 3rd rate shitty drama that always hit shoujo when they start to go out ("Lovely Complex" also had it, they simply used comedy to make it having less of a impact).
Harem simply make it easier to go into THE SERIOUS DRAMA! or comedy without the show running out of steam unless they use heavy guns like Incest (Koi Kaze) that is so fucking heavy it never runs out of steam.