since we now have image verification, to get rid of bots
email was added when we didn't have image verification. so it definately served its purpose when it was put in.
email, compared to just image verification, would marginally inconvenience a spammer. the img verification will stop bots, so it will have to be a person. i'd guess that even the average person probably has 5 email addresses? by the time u get anywhere near sign ups then u'd have IP blocking, and if they are going to the trouble of masking their IP then surely the email step isn't going to stop them? (or, they would just go somewhere easier)
u could say if it stops me bothering signing up to some sites, maybe the same will be true of spammers.. but its not like you'd advertise it so i would doubt that