I've never once felt I needed IS on my 50mm f/1.4, and I frequently shoot that at f/2 or f/2.8. I can't imagine a situation where I would need it at 35mm f/2. But again, as someone who shoots stills these lenses aren't for me, they're for videographers. They're the ones that are going to benefit the most from IS in a wide lens.
So like I said, it would just be smarter for me to get the Sig 30 1.4. Maybe there are some people shooting stills who would really want IS on a wide prime, bully for them if they want to lay out that much scratch, but I don't think that's going to be most people.
whatever floats your boat, better to have then not to have.