I have the following on 3 separate bikes, Tubus Cargo, Surly Nice Rear rack and Trek Interchange. I use Ortlieb back rollers.
1. Paint wear happens. But with regular use, the surface that has worn remains polished and there is no rust.
2. Once after a mild accident, I scraped the paint off the Tubus at some odd corner. It started to rust within a week. Bit of sanding the rust down and a little touch up paint cleaned it off.
3. I have rolled some electric tape at the contact points now and don't worry much about wear or rust.

I suppose you should go with what you like and have reasonable expectations on their function. All the racks I have serve their function and I don't bother with them apart from replacing the electric tapes when they tear.