Gravatar chose to implement their public API using the
spec ("Simple cross-platform distributed computing, based on the standards of the Internet.").
XML-RPC indeed is a simple spec to implement and is pretty easy to consume as well when the only thing you need is to pass XML back and forth using HTTP requests.
However, due to its simplicity its also not as structured as other specs such as SOAP or REST and so it can be more difficult to build a consistent and modular client.
This is at least the case for Gravatar's implementation where for example not all of the returned types are always consistent between different methods.
Gravatar.NET abstracts these issues from the client so the consuming app can expect a relatively similar approach throughout the API.
XML-RPC on Wikipedia