More robust connectors.
The capability for positive-lock connections.
and, of course, noise rejection if a balanced connection.
The only advantage RCA has is its smaller size (less space taken up on the backpanel.
I agree that there's not much lost if you don't use XLR; but if I have the option, all else being the same, I will every time.
Then again: I also like the speakon connectors.