I live in a rural area and there are huge grain fields all around me. At least for these kind of crops, the field is seeded 100%. There are no gaps for the tractor wheels. Having said that, you rarely see tractors pulling a sprayer in the first place anymore. Most crops around here are sprayed by purpose built sprayers that have tons of ground clearance, have relatively narrow tires, very wide booms, and are comparatively very light vs. a massive tractor. They can be built so light because they aren't used to pull heavy implements behind them. All they carry is the chemical, the spray booms, and the operator. Later in the season, it would be tough to pick out the path these things took through the field if you could at all. As for costs, the spraying is often done on contract so the farmers don't buy the sprayers in the first place: they pay for the service plus the chemicals.
For this kind of application, I think drones have a snowball's chance in hell of getting any kind of traction with farmers in the area. Their capacity is too small, their runtime is too short, the area they can cover per unit time is too poor, etc.