[[Special:AbuseFilter/21]]
Because he, along with myself and many others (think m:Global rollbackers, m:Global sysops and m:Stewards for example), are crosswiki users who do things on many wikis, including this one. Having a userpage on all wikis, including this one, is helpful so that people know who we are.
Also, discouraging userpage creation like this definitely does not encourage people to contribute to this project.
If you "do things on many wikis, including this one" then you would already have contribs and the filter wouldn't do anything. But that's neither here nor there.
The real question is which is more important: not wasting more of committed volunteers' time, or not dissuading that subset of newbies who create an identity before contributing from staying.
Well, there are plenty of more advanced abuse filters on Meta, Commons, English Wikipedia, etc. that I am sure people would be willing to let you borrow and that would be more accurate... and besides, the filter you've written won't stop mainspace spambots either.
For reasons unbeknownst to me, the spambot operators seem to have a fixation on new userpages. I'm sure they'll learn. To the best of my knowledge, we have gotten no offers from larger wikis to help us fight vandalism. If you can put us in touch with someone who wants to import more advanced abuse filters to en.wiktionary and suit them to our needs, I for one would greatly appreciate that.
Just borrow one of the more advanced filters, or keep some kind of whitelist or something. I shouldn't have had to post here twice just to create my userpage. That's just a waste of time.</pointless rant>
Right, because ridiculous user pages are less of a waste of time than stopping spam and we should obviously spend the time to maintain a whitelist of every active wikipedia editor.
People who actually contribute to this wiki have no problems creating a userpage. people who don't contribute have no business wanting a userpage.