Trust me- rust is bad. I pulled a 1997 E350 box van off the road. You could stick your finger through a couple of the crossmembers that supported the box.

After that, I never drove the truck again!

The undercoating is definitely worth it. As for using it with bed liner, that's an interesting idea. I'd also wonder what would happen if you put bed liner on the areas by the wheels. It's ugly, but I saw a Suburban entirely painted in bed liner. It looks as good now as it did five years ago.