Do the Yankees really need to do this? No. This team is already great. They have the best record in baseball, thanks in part to three starters with at least 10 wins a piece. They don't need Cliff Lee.
Or, do they? The Minnesota Twins, up until now, were considered the front runners in the Cliff Lee sweepstakes. The Twins can't beat the Yankees. If they acquire Lee, they can beat the Yankees. Looking at the picture from that angle, you can say that the Yankees do need Cliff Lee, to block the Twins from getting him.
I'm not a proponent of giving up prospects to get soon-to-be free agents, but in this case I wouldn't mind. Lee is a Yankee killer and that is putting it mildly. Plus, the Yankees have the ability to sign him at the end of the season. If I were Brian Cashman, I would go ahead and make this trade.