Microsoft OEMs will build them in order to qualify for discounts in desktop and laptop W8 licenses. And x86-based tablets will be able to go into classic mode (and JIT JavaScript) and run current x86 apps.

Why would anyone desire a W8 ARM tablet instead of a much cheaper (as in "you can buy two of them") Android-based one or an iPad is an excellent question, but we should never underestimate the power of advertising.