I think that it's more of a time saving thing. Those guys are trying for a show quality finish, as well as trying to fit into the segment constraints of a telivision show. It's quicker for them to cover the whole panel and sand it down than to do each problem area individually. They also get better results in final finishing because they smooth everything at the same time. Unless you have a large area that needs refinishing with filler, I'd do only what needs to be done. You'll find that you have to move quickly if you try to use that much filler at once, as it will set up rather quickly.
Personally, I get somewhat apalled at how much filler they use. I would think on a high-dollar show car, they would use good metalwork instead of covering it up with filler. But that's just my opinion.
