One less intermediary for the interaction. That's my understanding of the thought on the subject. The pimp has less economic and coercive power if a girl can make her own appointments. If a girl feels she needs protection it's a lot easier to hire someone to watch outside while she goes and does her work, but that is a very different position than that of the traditional pimp.

I find the argument convincing. Unsurprisingly there's not a ton of research on the subject, at least none that comes up on quick searches other than the usual stuff about violence vs sex workers decreasing with legalization.