Re: polymer bunk cover
When I purchased my trailer in '05, it came with the "plastic" bunk covers. I figured they'd be hittin the dumpster pretty soon.
BUT
I love them - they seriously cut the friction. It's like having a roller trailer, but with better support. As for paint wear. My boat is bottom painted, ablative, so after more than a couple hours in the water and it is soft enough to rub off with your finger. It is not wearing off any faster where it rides on the bunks than anywhere else. Although, I rarely need to winch more than an inch or two, so your mileage may vary.