Whizz wheels don't run out of batteries or get knackered when you drop them. They are very reliable and give surprisingly accurate answers. Once you've got the hang of them, they can be just as fast (if not faster) to use as an electronic widget. They also require you to use your brain, and you'll find that your mental arithmetic will improve when you've been using one for a while.
Why throw technology at a very simple problem that can be solved using a couple of bits of twiddly plastic?