Some people don’t use Gravatar, some don’t use Twitter and there are some who don’t use Facebook also and when they leave comment on your blog they leave a mystery man image (or your site’s default avatar) in their comment which sometimes does’t look nice. But you can allow them to choose their comment avatars manually by selecting a avatar from their social profile, email or choosing it from a list of your site’s custom avatars. I know about few plugins that allows you to do so.