Matt - I've said the same thing about Arbitron, as well. Depends on where the books are dropped, then who fills 'em out. I've seen a station (or multiple stations) do really well in the Spring book and then tank in the Fall book...and in that same Fall book all of the other "like" stations tank, too. So, it looks like everyone decided that they now hated Country and really liked Classic Rock (use whatever format you like for the example I just gave), and we know it's not like that. Mass amounts of people don't suddenly change formats in a market.
I like your online idea, however the idea is supposed to be that one carries the diary (or device in PPM markets) with them everywhere they go, so they can record the station they're listening to right then and there. Yes, laptops and smartphones exist, however you can't count on everyone having access or compatibility to the online version everywhere they go. That being the case, the "habit" of carrying and recording what you listened to isn't created, so the person no longer thinks about what they're listening to, they don't have a reminder with them (diary). So, IF, and I mean IF, they go back to their computer at the end of the day, they have to try to remember what they listened to, and then they'd just select a full day of their favorite station whether they listened to it or not.
And, like you said, a good salesperson can sell the station, if they know the product and the target. National buys go by numbers in target demos, local buys usually go by what they perceive. In that case, the numbers don't matter. They're gonna buy on their favorite station, or the station they think is best for their business, or, in some cases, the station that has the best salesperson who can educate the client.