I too, do NOT, have a true back up bow. I do travel with a spare set of limbs and string for my primary hunting bow.
I do have a couple of other bows that i shoot regularly and that shoot with the same arrows. One reason i dont have an exact duplicate is, well, quite honestly, if i want to spend money on a bow, i would rather have something different.
I have never needed it, but having a bow as a back up is a great idea. I would say, however, you have to think over your hunting situation.
If you are hunting a local woodlot and can get home in a reasonable amount of time, then i would only bring a spare string.
If you are driving a considerable distance, or paying big bucks for an "out of area" hunt, I would bring another bow, or two, maybe even three.
If you own a takedown, work the classifieds for a spare set of limbs.
Good luck and let us know how what you decide.