For some more background, when we proposed something like that to Chris Steipp he was pretty iffy about it, and he's not wrong. At other sites
that
don't have a CAPTCHA on signup (like Facebook, Quora, others) they avoid a spam problem in part because they require an email address and confirmation. For some irrational reason, even in the era of throwaway email accounts from web services, not requiring email is some kind of sacred cow among Wikimedians, even if it would make it an easy choice to throw away our wretched CAPTCHA.
Because spam bots can't answer email?
If we want to avoid spam bots signing up, there is going to be a hit in ease of use somewhere. Our network of sites is just too large to avoid being a target. It's just a matter of testing to see how much we can
reduce
that hit, and which method might be less easy.
There is really no evidence that the current hit in ease of use has any affect on warding off spam bots.
--bawolff