gravatar is a useless service which has no place in core. It identifies email addresses and not humans since there is no verification attempt that the human actually used the email address to comment, therefor it is not a true personal avatar (you are not in control of its usage).

Then there is the "global" problem with it. I can't use the same email address on star wars site and harry potter site if I want a customized avatar for each. just stupid.

It is an wordpress.com internal service and as such probably does what it expected to do there, but for hosted wordpress its usage is just yet another example of how little love comments get from core developers. In 4.6 we at last get rid of google tracking us on our site via fonts. maybe one day we will be able to get rid of automattic as well (don't hold your breath)