The agencies, local or national, have the ratings. They subscribe and get the data at a very reduced cost, subsidized by the subscribing stations.However, that station's competition reserves the right to take those same Nielsen reports to those same advertisers and use them to make a case for moving advertising to them. A station that cancels Nielsen has to be confident in their relationship with their local clients, and that their relationship can withstand the salesmanship at the competition.
So even if you are not subscribed, agencies have your data. But you can not do all the calculations and presentations agencies often need. So you lose an "edge".