Google says it’s taking on comment spam.
I’m not an expert at this type of thing, but it appears they are just taking a step to prevent their web scanners from reading the comment spam; it won’t hinder the spammers in generating the comments to begin with.
The point here is that comment spamming isn’t directed at the readers of your blog, but it’s a way of goosing the Google page rank of the target links entered into your comments by the spammers. Basically, they reason, if Google doesn’t use links appearing in blog comments in determining the page rank of pages indexed by Google, the motive of these jerks to comment spam will be significantly reduced.
Of course, that assumes that comment spammers are rational actors, which may be a dubious assumption.