That type of thing would probably wind up being way too limited. It has to be generated and checked by a computer so it's going to tend towards a pattern and no

That type of thing would probably wind up being way too limited. It has to be generated and checked by a computer so it's going to tend towards a pattern and not be arbitrary enough. And as you make it more convoluted you're probably going to piss off the users more than slow down spammers.