yeah, It's more to do with slight upward curvature whether you hit the g-spot....
BUT, alot of women don't have a g-spot and if they do it's only 3inches in.... and wheres the clitoral stimulation in that?? do you really wanna be just 3inches in after workin' so hard for gains?
It's prob. best to mix it up deep n' shallow for most gals.