I don't know. The sharpening defaults are pretty aggressive. I created a preset where I back it off to about 0.5 to start, as the sharpening at 100% is quite strong with the defaults, at least on my 30" Cinema Display.
That said, you can create your own preset if you want. Defaults are just a suggestion if you do nothing. I have an "add punch" preset which adds sharpening at 0.5/default/default and then 0.2 for define, and it's a decent starting point.
I do with there was some good documentation around Aperture's sharpening. It's not really well documented.
What makes it even more confusing for me is, I sometimes will apply sharpening over in one of the Nik plug-in apps, where I have the sharpening vs structure options.
I agree: more info on this subject would be helpful. The Fraser-Schewe book Real World Image Sharpening explains how sharpening works in Adobe Photoshop and Lightroom. I think the more general early parts of that book probably pertain to Aperture, too. But I'm not sure how it all works out in the end. If the sharpening is critical, I work on my bigger monitor, try to guess whether I'm doing it right and try especially hard not to overdo it. But sharpening remains something of a mystical art.