Right, it doesn't make sense if you are replacing Lester, I agree.
But, if Lackey has a solid first half and you feel that you will be able to trade him in the offseason, it would improve the rotation substantially.
Or, if you trade Doubront in the deal to the Phillies, and trade Lackey in the offseason, you can have a rotation of Lester Buch Lee Dempster Webster, which would be crazy good.
This all obviously has a lot of ways it can go, depending on health and performance. It was really just a question in theory, if there is a need in the rotation, would Lee be a real option, and I think he would.