SUPER-wides on rangefinder are easier to focus than in an slr, and the image is brighter since the slower wides are a bit dark. This is an important consideration.

I've used a leica 21 and a CV 15 for years and never noticed much problem with parallax error. If something is real close -- the leica focuses down to 18 inches or so -- you just eyeball it a bit, but they're so wide that parallax error is't too much of an issue because the error will be a small percentage of the whole area being shot.