I doubt the lakers would want to trade bynum,he's one of their best defensive players and why would they trade him if the lakers are playing bad because of their defense?
Because Melo is hugely superior to Bynum, irregardless if Bynum happens to be classified as a "defensive" player.
So, they trade Bynum. Their D goes down by a factor of X.
But their offense goes up by a factor of X + 1
The latter scenario helps their team the most. Even if their current weakness becomes more weak.
IMO.