The pros do it (fast) because they get paid to do it, whether on TV by advertisers where they are really in a hurry, or in a shop where the idea is to get the one your working on out, and the next one in. Think about your job whatever it may be. If you sell bark for a living, then you will put it where the customer wants it, but probably won't spread it and arrange it quite as carefully as if it were your own landscaping.
If you have several dings, or dents in a confined area, then sure catch them all at once, but I still say patients is the best body tool you can have. If you're in a hurry and don't care if you have a little more filler in your finished product then I would go with mass-filling. If you have time and want a really fine product then panel filling wouldn't be the best method in my experience.
I think that we are over looking one thing here also and that is most dings and dents should be straightened, not filled. Careful work with body tools and a little heating and shrinking of metal is a far superior fix to filler.
I'm certainly humble in my remarks, but as a perfectionist when it comes to my projects, I would rather spend a day on one panel if need be and have it right, than to fill it quickly so I can move on.
