That's not that simple, actually. Drain holes by themselves are going to allow moisture in to the car as well, and being holes the edges will need to be protected, usually drain holes will have some rubber insert for that. But then they are no longer flush, so some water will pool. I've welded plenty of car bottoms where moisture had done a very thorough job of rusting things out, drain holes are typically a fantastic way to get a problem, rarely a way to solve a problem.
Some convertibles have this done the right way, but there the whole car is designed from the ground up for this, and that includes the upholstery, the electronics placement, the way the cables are routed and so on. If that isn't the case your best bet is to keep moisture and fluids on the other side of the bodywork.
Everything on a Boeing airliner has drain holes in it, usually at least 3/8" in diameter. Boeing really does not want accumulations of water in its airplanes, as it causes corrosion and can form ice and it's heavy.
The drain holes in car doors are usually too small - they'll get stopped up with bits of grit and leaves. I drill them out larger for Seattle weather.
Aircraft are not typically made of steel. Do make sure that you coat the edges of your drain holes after you drill them out with some sealant (epoxy coating for instance) so that you don't give rust a fantastic place to start and make sure you get all of the shavings from the inside of the door.
I didn't worry about the edges, as it would take a very long time for the corrosion to progress that way on sheet metal. Didn't worry about the shavings, either, just more of the usual grit at the bottom of the door.
Though if I have a dab of paint or grease handy I'd use it.
As the former owner of a metal workshop I think I'm aware of that little detail :) Especially fun when welding it without fancy processes. The speed with which that top layer oxidizes is uncanny, you can actually see it happening under a microscope.
I've seen tiny little holes in sheetmetal cause enormous trouble in no time because it causes the paint to blister, lifts it up and then exposes a much larger area, this is an exponential process, with ever new bit exposed an even larger surface becomes active in the process because the edge length keeps increasing.
MB had an interesting mishap when they switched paint processes, it caused absolutely miniscule holes in the paint, the result was entire doors rotted out in under 3 years, resulting in massive recalls. Once rust has a hold on something if unchecked it will progress rapidly.
Basically you're right, but I didn't have any trouble over it. The holes let the water out and allowed the metal to dry. This was drastically less of a problem than having the water sit in there. Even worse was the mud that accumulates in the bottom of the door would stay damp, pressing the moisture against the metal.
Holes not only let the water drip out, but help to dry out the metal.
I notice dramatic differences in rust on metal where it can dry out vs where it can't.
Yes, but doors have a very nicely defined lower point without a whole lot of variation in the profile of the bottom bit. Even so, they rust out frequently, as the sibling comment to yours mentions, in part because they tend to be too small and get clogged, in part because the moisture running down the inside of the door can still sit on the level parts for a surprisingly long time, and soak into the sound proofing and between the sound proofing and the various metal door panels. Depending on how porous the paint is and how good the adhesive is it could get problematic (there was a time when car manufacturers switched to water based paints and cars from that early period after the switch tend to rust like mad because the paint was more porous).
Finally, keep the door rubbers at the top maintained well, the best way to deal with water is not to get it in there in the first place (but moisture from condensation is impossible to avoid).
Some convertibles have this done the right way, but there the whole car is designed from the ground up for this, and that includes the upholstery, the electronics placement, the way the cables are routed and so on. If that isn't the case your best bet is to keep moisture and fluids on the other side of the bodywork.