Sweeps months really exist only for local stations. For four times a year, it is the only way local stations can gauge their audiences and then set their ad rates accordingly. To top it off, the U.S. election was held this sweeps month. The results obtained from November sweeps don't fool any media buyers. It's simply a starting point to negotiate rates.
Media buyers are very aware of what sweeps months ratings represent.
In fact, one-hour serialized dramas have the worst rerun audiences than any other type of show. Things like this are all factored in when you're buying space in the off-months. And any buyers who don't know these facts won't last very long in this business. Well, perhaps airing a controversial mini-series or a celebrity driven show will attract new viewers to a channel that will likely hang around the rest of the year. Brill, for one, would like to see a better year-round consistency.