Matt Cutts said that generally most links are DoFollow on the Internet. I doubt he was talking about blogs for that comment.
Wikipedia is a no follow website. Wikipedia is experimenting with ways to make some links DoFollow, but if they did that overall, there would be thousands of entries made just to get the backlinks.
If you are under attack from spammers, it's a good idea to add the no follow tag. Otherwise, don't worry about it.
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