The thing about it is that, if it is just to carry spare wheels, it is a LOT of extra weight. Pounds, not ounces.

To me, it would make more sense to carry a spare tire (foldable type) and MAYBE a couple spare spokes, taped to the bike somewhere.

Otherwise, you add so much weight to the bike that you're traveling at walking pace → may as well just walk.

No? What am I missing?