If you're a perennial playoff team with an elite QB and you want solid backup in case disaster strikes that won't derail your drive for a SB, I can see why you would want a guy like Matt Moore around.
But we are not that team and we can't yet say for sure if we have an elite QB. One thing is for sure, Matt Moore is not an elite QB, so I don't see the point of him on the roster. He's a mature, happy, chirpy security blanket for a team than needs hungry motivated highly-talented competition for Ryan Tannehill.
Forget about cap room. We should ditch him and probably Devlin and sign a new QB every year till we have someone breathing down Ryan's neck for a start. Even a late rounder or a UDFA, but preferably an occasional mid-late rounder with big upside.
The perceived benefit of a safety blanket QB is overrated. Plenty of teams do OK throwing in rookies and plenty of teams still struggle when they roll out their vet backup.
Moore has trade value. You could probably swing a late rounder and replace him for no loss of picks.