Yes, definitely. Or maybe I should say maybe.

 Done properly, putting clear over trim and graphics will definitely help things last longer. I do this on practically all of my models. Depending on what products you are using though you must be very careful not to ruin the vinyl or whatever trim you're using. For instance, if you go too heavy right off the bat with the clear, the thinner can and will easily attack and wrinkle the vinyl. This will really ruin your day. You can get away with it though by fogging on several light, dry coats, and letting each of them dry as you build up to finally going with a couple of heavy coats at the end.

Depends on materials. If you are talking Monokote and Trim Coat then you may or may not get adhesion. I had some success with shooting catalyzed polyurethane over iron on plastic coverings, but I've also had it peel off. Better luck if the covering is something like textured like Coverlite.