Bargnani is actually a very good defender at his position, since he was moved from C to PF and got a defensive minded coach. His opponent PER according to 82games.com went from 21.3 in 2010-11 to 14.3 in 2011-12. This stat also passed the eye test. He can stay in front of most PFs and he has great reach.
However, this proposed trade would leave a huge hole behind Pierce at SF. And I would rather keep Sullinger, than build around Bargnani after the end of the Pierce/Garnett era.
He actually is a good low post defender. Knows how to use his size and long hands. The problem is his help defense. He seems to be always lost when rotating. His pick and roll defense is also bad. And his rebounding is ughhhh.
I wouldn't really mind having him. I think if he's utilized properly he's a matchup nightmare for some. But not right now, we need someone who can help defend as KG couldn't not do it as we'll as it is before. We need a rebounding forward, Bargs is something we don't need righ now.
And even if we need him, I won't so it for that proposal. I'll keep Sully and Melo.