What makes a good golf swing?

Does a golf swing have to be on plane throughout the motion to be good? For instance Jim Furyk has a unique swing that is not always on plane. It is a swing where he takes the club out and above plane on the way back, and then drops it inside on the way down, and manages to get the club on plane and the clubface square at impact. Even as good a player as he has been for many years, he has worked very hard over the last few years at getting his swing more on-plane. To get the club at impact the same everytime and repeat that impact time and time again is most easily accomplished by an on-plane swing. This more on-plane swing seems paid off for him in 2001 as he improved to 17th in greens in regulation. This is the way to lower scores.