The side of my 4x12 is getting scratched because of how it has to slide into my band's van. Would it be a problem if I covered up the side of the cab with duct tape? We don't have much room and I would like to avoid buying a whole case.
Yeah, the duct tape will, at some point,either refuse to come off, pull the top layer of the tolex with it, or will leave behind a residue that will be nearly impossible to remove. Those are some of the problems (not including the "it's going to look like hell" bits).

A smarter solution would be to screw a set of "rails" onto the side of the 4x12 that's affected, something like what's done with this 810 bass cabinet:

An even smarter solution would be to strip off the tolex, sand it down smooth, and have your local shop spray the whole cabinet down with LineX pickup truck bed liner. It's nearly structural (check out some of the Mythbusters videos), looks like tolex (comes in colors, too) and can withstand brutal handling. It's easily repaired, too. I'd still put the rails on.

I have two 2x12s that have been run through the mill over a period of four years. They look close to new. Both were sprayed with LineX when they were built.