Although I have often backed up a semi with a wheel-gun, your first choice for a back-up should be one that functions like the main gun. Having it in the same caliber is even better, and the best would be to have interchangeable mags.
But I’m certainly not against wheel-guns. One specific application for using a wheel-gun as a “back-up” (although it really wouldn’t be a back-up, in this scenario) would be having a hammer-shrouded unit in a pocket. The shrouding prevents the material getting in the way, and the simplistic action of the weapon itself allows it to function (unless you’re wearing really big clothing, there’s probably not enough room in your pocket to cycle a semi).