This would deter more casual vandalism than any suggestion I have seen and be of trivial consequence for genuine users. Is there really a good reason we don't do this?
Zero.
I think the problem with this is that there are too many false negatives and false positives--too many anon edits which are solid contributions, and too many logged-in users who use their accounts for vandalism. I think there's only one reason that there are so many calls for this: most of us never actually see Wikipedia as it was meant to be seen. Out of 200,000 articles, most of them are pretty good--it constantly amazes me the sheer mass of information. The problem is that most of the things on Cleanup, VfD, and Recent Changes are the things that need work. Thus, regular users get very biased views of Wikipedia, since they only see the very worst of it. As a resource as a whole, the problems are not very severe. Just my two cents, anyway.
Meelar
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