It's a good point you raise and one I think about from time to time.
The counter argument is that you could probably say the same for any player. When any 3 players are "on," Melbourne will look better.
For me, the question comes down to consistency and how regularly they can play at or near their best. The players mentioned in this thread fail to do so anywhere nearly enough, but they're certainly not alone.
On any given day I'd be comfortable that Frawley, Jones, Watts (bang!), Bail, Bartram would play to that level in terms of their own game but not many of their peers.
It's a fickle game, footy. Two years ago Davey would've been one of the few on that list. Last year Green would've been on the list with quite a few other mates.
Hopefully, that list will increase again - because if it doesn't, we're not going anywhere. (I'm not saying that list is definitive, or even correct, but I think anyone will get the message).