For me Lensmannn, it's about symmetry and balance. If you're working with a long, wide blade then the thickness should match it in order to be well balanced and plenty strong.

Let's take a bowie knife with a 10" blade. At the ricasso, the blade should be no less than 1 1/4" wide. And the spine should be at least 1/4" thick. A knife that size using 1/4" stock and fully forged or ground from the spine to the edge and with a full distal taper should be extremely light in size to weight ratio. I'm talking about a flat grind here, of course. A convex ground blade will be slightly heavier.

If the width, length and thickess aren't matched to the size of the knife, it'll be out of balance and just won't look or work right. That's my take on it anyway. I use alot of 1/4" stock along with some 3/16" too. But mostly 1/4".