Roberts is a pretty crummy defender, but he's still rated out better than Rivers in DRtg and DWS last year and through the early part of this year.
Clearly I am a numbers guy and Roberts is the superior player by just about any on court measure.
However, I do think Rivers can play a role on this team. He brings energy and enthusiasm (if not always competence) which this team needs. That is valuable and also immeasurable.