They just about never get answered, and when since they always involve posting email addresses there have been several times where some who bumped a bunch of times gets a reply like "Thanks for posting your email, bumptard. I just signed you up for gay porn!"
Also users report them as if they were against the rules...
as long as the E-mail field is on every page, posting e-mail addresses should never be a reason to delete posts unless they are obvious calls to invasion. if people are too stupid to use yahoo or spambox or the multitude of free services out there and post their real e-mail on 4chan, they can't really complain about anything.
also i see it as a legitimate request. most of the time the invites are for private torrent trackers like Demonoid and if we start killing them from /r/ they'll just go to /t/ and they'll still get deleted. so i'm with >>985 on this. let them be as long as they stay civil, but delete the second it turns flamey.