One advantage of Gravatar versus local uploads...........One site I manage for someone.........requires people to have an account to comment and participate.
350,000-ish accounts.
No login via facebook, twitter, google+, linkedin, etc...

If each avatar of those accounts is 100kb.........35GB. Just in 512x512 PNGs/JPGs.
If you have 50kb...17.5GB.

I am all for local and avoid third party connects (hi akismet, jetpack, google analytics)....but everything has PROs and CONs. local avatars have the above con.