Come take a look at my truck if you want proof - the only rust on the entire chassis is from that AC condensation drain. I live in San Diego we get about 1" of rain per year pretty conclusive evidence that " a little drip of water " can do a ton of damage. It's not just a little drip it's a small puddle every time you park and when you live in a warm climate and run your AC every day it is water boarding for your frame...
I'm sure in a desert environment this would be one of the biggest causes of rust on the truck, i'm not disagreeing with that. What I am disagreeing with is that this is an issue. If a truck can last 20 years up north in what we put them through, that one slightly rusty spot will not cause your frame to fail in that same timeframe. I'm doubtful that puddle would even start to cause moderate scaling after a decade.