...and a fisheye lens isn't? [a-one-trick-pony]
You are right it is. My point was that an ultrawide rectilinear zoom lens or a fixed fisheye on a cropped sensor camera are both one-trick-ponies. The Sigma 15mm is very close to rectilinear when you factor in the crop of the APS-C sensor.
There are many philosophies about how to assemble a lens collection. You seem to be passionate about the benefits of ultrawide zooms. I am trying to show the OP other options. For a similar or the same price as a 10-22, you can get a very good ultrawide prime that is faster and possibly slightly better quality.
There is no right or wrong.