Wikipedia uses nofollow to discourage spam edits. Google and other search engines are free to ignore nofollow and try to assess link quality in other ways.
Editors are usually pretty quick at removing spam links. But yeah.
At the current level of spam link posting, sure. But if getting spam links into Wikipedia were more valuable -- say, if they stopped using rel=nofollow -- I wonder how much the amount of incoming spam would increase.
I would be happy to know they tried, or at least considered it. Maybe they have and I'm just not aware of it. It's just something that always bugged me a little.