How thick of metal for frame connectors

I noticed that nobody puts holes in their frame connectors. The factory frame rails have holes in the bottom and sides. Is that to drain out water, or to allow coating the insides? I always worry about rust, so you might consider painting the insides of your frame connectors, using zinc primer at the ends for "weld-thru" protection. When replacing my floors, I washed and blew the front frame rails out and spray painted inside as best I could, and pulled a primer soaked rag thru (Rust Destroyer).