@absence: this was mentioned here, but I'm not sure if any consensus was reached:
viewtopic.php?f=19&t=1420
I have no knowledge myself, but I had always just assumed that the pots which were made for drinking from the spout were the ones shaped like this (which is not a shape that's very common anymore):
I don't know if that was one of the reasons for the XiShi shape, though (and of course, XiShi teapots also have their own 'history' / legend, much like the fable about Marie Antoinette and the champagne coupe...).
Hopefully someone knowledgeable can add or subtract from what I've said.
And as to your original point about the ergonomics of teapot knobs, my personal view is that a small, round lid knob is simply the best shape for brewing from a small teapot using one hand, as well as being simple to make, and rather elegant on a small pot. Brewing from a 40mL teapot yesterday reinforced that notion in my mind.
That said, like the others, I've never really had any problems with the design (although if you're picking the knob up with three fingers, that seems far more dangerous to me than picking it up just with two, or with two fingers plus one holding the edge).
Andrew