I can't speak for the usefulness of the caps, but I can for the nut thing. Once I lost the nut on a tube and just didn't worry about it, then a few months later when I got a flat, I took the tube out and there was a ton of oxidation and corrosion around the stem. And the other tube that had the nut in place had no corrosion at all.

I imagine that will eventually cause the tube to fail around the stem.