Like Tinman, I guess I don't see a whole lot of harm in a provider posting renewal ads multiple times in a day. A relatively minor nuisance, at worst, and probably not much different from the previous ad board scheme in that respect.
One other little thing: Some, rather than a specific time range of availability, write "available anytime." In what way does that differ from the previous ad board format? If a guy contacts her, any advertiser could simply say that she just filled today's scheduling window and ask whether he'd like to plan for some other day. That wouldn't fit in with what's ostensibly the purpose of an "available now" forum.
But since some gals travel a lot, there may be value in that the forum at least gives some idea of who's in town and motivated to see clients.