The problem with harems is that they limit the personality of the girls involved to the type who like that sort of thing. That's an extremely small subset of the full range of personalities available. Some of us happen to like girls who, for example, need an emotional bond to feel comfortable with a sex partner. Strong characterization is one of the keys that differentiate an AVN from generic porn. If the relationship feels no different no matter how many girls are involved, then IMHO the relationship is irrelevant and the characters fake.
The other problem with harems is that they kill replayability since there's no need to play different branches if the content never changes, but that's a separate issue.
As far as The Arson Betrayal goes, however, I see the point as somewhat moot. The game explicitly gives us the option to turn down girls, so it doesn't really matter who would take that option or why they'd do so. The game has already chosen to accommodate them. The question is whether the game is getting as much mileage as it can out of those options now that they've been added, and I say there's room for improvement.