My roommate does not multibox, but uses the ascii characters frequently.
He's a huge transformers fan, and without the special spelling, it would make it impossible to get some of his names.
Megatron, Soundwave, Laserbeak, etc.

It's not just boxers who use it.
Given we're a smaller percentage of the wow population overall, I'd imagine we're a smaller percentage of those with the funky names too.






*Edit*
Pretty much ascii characters violates the naming policy.. but GM's don't actively enforce this.
If someone on your server reports you to a GM, via a ticket, then you will be asked to change the name(s).
So its policing via the community, without customer service actively looking.