Is a flat swing a bad thing?

There's a fascinating picture (it's somewhere on one of my computers) showing 9 great players on the way "up" and on the way "down". Then, someone technical person has measured the angle of the club and the difference between the two.
With a one-plane swing (or close to) the difference is very small, only a few degrees....think Vijay....with others, quite a surprising amount.
Matt Kuchar doesn't appear on this picture, which is no surprise since his positions on the backswing are not exactly orthodox, and it's an old picture.

To a certain extent, it's only about setting yourself good at the top to come down.