(UPDATE: The Mavs have in fact added Melo and two other players to their roster in anticipation of training camp.)
Per Marc Stein:
Fab Melo was released by the Grizzlies just two weeks after a trade sent him from Boston to Memphis. The former first round pick out of Syracuse appeared in just six games for the Celtics, and failed to make waves in Summer League action. If you're wondering how crushed Melo was, having been discarded by two teams in less than 24 hours and being the first member of the 2012 first round to be waived...well, he wasn't that crushed.
Back in 2012, there were some projecting Fab Melo to Dallas, as the Mavericks were(and still are) in need of big men. Given who Dallas ended up actually taking in that first round, it's hard to say they would have been so much worse off.
Melo is a legit 7'0, and moves fairly well for a man his size. Though he can block shots, and generally take up space, he has not demonstrated much of any functional NBA ability yet. For a franchise that typically values basketball IQ and hustle, Melo is a strange match, but given his physical attributes and pedigree, he's at least worthy of a chance to make the team.