I’ve argued for a long time that having anonymous contributions is a good thing – even with the increase in spam, there is enough legitimate content to make it worthwhile. Plus – once a user has contributed, they are more likely to register and contribute even more.
A recent blog post by Topix validates my argument (huzzah!) The one surprise (for me) was that the amount of spam by non-registered was only 50% higher than registered. My experience is more along the lines of 100-200% more.
Of course there was no qualitative measurement done, but we could argue that in general about UGC Skrenta also has some good takeaways from the #s.