Melo does not make the C a contender able to beat Cavs, Spurs and GS. On the other hand, Melo's salary completely shuts down the possibility of improving dramatically in free agency during next years. Signing him would be a huge mistake. period.
Who's Boston going to sign this summer that's going to make them better than Golden State? The fact of the matter is Boston's going to need to trade into contention - and Carmelo helps this by making Crowder and Avery Bradley easier to part with - or just punt on the next couple years and try to develop young guys drafted through the Brooklyn picks, which Carmelo doesn't affect either way because he wouldn't cost a Nets pick in trade. He's not a cure-all for the C's problems, but if you can get him at a sufficiently inexpensive price there's something to be said for a non-contender just adding more talent.
Trading for Melo isn't a no brainer because he's aging, had injury issues, will eat up all out cap space and has a deserved reputation of a ball stopper who expects the entire offense to be him shooting jumpers. If we were talking about giving up serious assets for him, that's an easy "no."
But he's still good enough to make this team a lot better and if the price is...
1. A bunch of guys who are on the roster specifically as trade bait for someone like Melo.
2. A 2018 first in the mid 20s and a future first that could become a couple of seconds.
3. Filler that depends on what else the trade involves.
...it might be time to remember a bird in the hand is worth two in the bush.
Mike