In reply to Julio Potier.

@Julio Ah, p*****, I didn’t know they do. I currently have 797 spam comments in my database—not many, but from looking at every one of them I can say not a single one has a Gravatar.

I’d still say yes, by all means, do activate the feature and see how it works for your site. ;)
It might work very well, but if it doesn’t, there are additional counter measures one can put in place such as the free Blacklist Updater plugin that will automatically update your comment blacklist from a public source.