Point is that you can no longer use the url's hostname as a document feature for spam classification. You have to actually fetch (or at heast HEAD) the url itself.
This is true, but it's also true of any site that allows user-generated content. I would assume that it's easier to detect that bit.ly/gibberish leads to viagraaffiliatescam.com than to detect that randomblog.blogspot.com is a splog with 400 links, one of which goes to viagraaffiliatescam.com.
