It's not about Perkins and how bad or worthless a player he is. Everybody knows that's not the case. It's about the situation in whole.
Perkins didn't sign the fair contract offer. He's coming off major knee surgery. And you have a combo of Shaq/JO and now Krstic filling his position. The value of Green off the bench to go with a future Clippper 1st rounder can't be weighed right now, because we don't know what comes of it. Regardless, Green looks like a huge upgrade and adds scoring for the bench on paper.
I thought it was a great trade. Your basically saying Shaq, JO, and Krstic can be just as good if not better than Perk, and Green adds to your bench immediately. On top of that, you get younger and add future picks to help you out later as well. When you have a stacked roster going in, it makes it easier to make trades like this. This is a better team overall, and you get a 1st rounder to boot.