Yes, I've always heard to water before a rain too. But the reason I heard was because it makes the roots grow deeper. Don't know if there is any truth to it, but I always thought that it makes sense. Of course for some plants that wouldn't work well. Your reasoning sounds good too.

What a wonderful point to bring up! Yes, watering before a rain is a great idea. It's all about the forces of adhesion and cohesion and the way water moves through soil and is bound to soil particles -- or repelled by them. This year more than ever before, I've noticed my dry soil repelling the water I apply to it, like pouring water over baby powder -- or sphagnum peat. Once the peat is slightly wetted, it absorbs water more readily, but it's hard to wet initially because it is completely dry.

Rain water doesn't just run off dry soil because of a hard upper crust. It can percolate straight down past the top layer of soil because it's basically beading up and rolling away from the soil particles. So I always like to moisten the soil before a rain, to get the maximum benefit from the rain!

The slower the water enters the soil, the less "gravitational" water you get, and the soil becomes more evenly wetted where you want it wet -- at the root zone. That's why a light steady rain is optimum -- and a drip irrigation system is a nice substitute.