I use the method to get an image with the subject at maximum sharpness and yet with maximum amount of background blur.
The mushroom in the first picture isn't very sharp because the lens isn't very sharp at f1.8, even when the focus point is spot on.
As you probably know, almost no lens is optimally sharp at its widest aperture. Not only are they not optimally sharp, defects such as chromatic aberration are also more pronounced.
Neither are they optimally sharp at very small apertures such as f22 or more, due to diffraction. (They do of course give the farthest DOF, but that is not really important in this case).
Most lenses are sharpest somewhere around f5.6 - f8.0 (as a rule of thumb, this may be debated forever).
Taking the shot at f2.8 as you suggest would surely be a nice compromise, and useful for most situations, however it would neither give most background blur nor most sharpness.
Therefore I find the best way to achieve this is to merge a wide open shot with one at the lens' sharpest aperture.
You can see the effect more clearly here:
http://www.magiclantern.fm/forum/index.php?topic=3113.0Edit: By the way I didn't mean to sound so lecturing, I just tried to explain it as clearly as possible to anyone reading this who may wonder the same thing as you.