I think the point is you'd be increasing the wear. First, even in a matching set, no tire will be exactly the same circumference just due to variability in tire pressure and wear pattern, so you're always in this situation where there's some strain on the diff. The larger the difference, the more the wear. Too much difference and you'd be significantly increasing diff's temperature, which reduce oil viscosity and effectiveness, and subsequently result in wear on the differential itself. On an AWD, this a size difference between the front left and right wheels, the rear left and right wheels, or the front pair and rear pair wheels can cause excessive wear.
As to bends in roads, yes we have those too. That said, I would venture that 90% of all driving is still more or less "straight."