A couple of points: Michael is correct that 3 gallons will cover 12 full grown trees to run-off, if run-off is required. However, full coverage is more important and so ensuring that the material in the tank essentially "mists" the tree so that droplets of an appropriate size land on an optimal amount of tree surface volume requires some "energy" behind it. This is where the mistblower comes in.
In advanced sprayer technology droplet size is controlled through a combination of pressure (often greater than 150 psi) and nozzling technology -- none of which the mist-blower has. The mistblower has what I can only describve as a reverse Venturi effect -- the vortex of the air stream and length of the tube physically breaks up the liquid as it released at the end of the tube, in effect mist-ifying it (!). The nozzles on the end of the tube do have some impact in terms of directing the mist and defining the spray pattern , but in reality have little effect on the droplet size. Nonetheless, the effect the machine has on the droplet size and the force behind the deposition ensure good coverage throughout the tree canopy -- even in reasonably tall trees.
To Michael's final point, in conventioal ag, the idea of a "smothering" spray (e.g., oil) comes from the reductionist theory that creating a "clean slate" orchard environment is optimum. The fact is that a 2-3% petro-oil spray kills (smothers) everything it hits, including beneficials. This is antithetical to a holisitic orchard. Good coverage does not mean killing everything in its path -- we live with the reality that there will be some good bugs and some bad bugs and the two will balance each other...but int he end we have a healthy, vibrant orchard environment. All we're doing as holistic orchardists is assisting nature (persumptious as that sounds) so our trees can grow a crop for us.
So, I don't know if this helps or hinders the conversation -- but that's another of my 2 cents thrown in. However, I don't think run-off in the strict sense is necessary with every spray, but good coverage (tops and bottoms of leaves) is. This is wht I like the mistblower vs a handpump sprayer, if you can't afford or don't need a small airblast sprayer.
Mike Biltonen, Know Your RootsZone 5b in New York