The trade-off is supposed to be that you can't
always get a last-minute room at the Four Seasons when you book it for "tonight" -- sometimes they'll be sufficiently full that they won't have sold that block to HotelTonight, and instead of having a plush room right across the street from where you'll be working early the next morning, you'll need to settle for a "motel" right on the highway, 10 miles away.
I'm not sure how these things end up playing out (and if HotelTonight shows data that maps how often selected hotels actually have rooms available, that might undercut the hotels' stance a bit), but I know I wouldn't want to rely on it; even planning in advance, in my experience it's not terribly rare that hotels are booked up if you don't call early enough (and it's an in-demand area).
Edit: ah, yup -- that's HT's pitch to the hotels. From the section for "hotel partners": The key to targeting these unique and valuable guests is the IMPULSE DEAL, a rate so you good you only use it when you absolutely need to move rooms at the last second. Since you don’t have that need everyday, our display changes everyday, meaning guests can never predict when your hotel will be offered.