What does the hull shader attribute
[patchsize(12)] do? MSDN fails to mention it. Do I have to use it? Should I use it?
patchsize appears to describe the number of control points per patch, and seems to be largely ignored (as unnecessary) by the actual compiler. It's possible that at one point it was used as an optimization hint, but doesn't seem to have much practical use.
All real-world examples I can find eschew the attribute. The fact that it appears to only be referred to in the documentation here suggests that the tech writer who created that documentation was given an old bit of example code.
I'd say it's safe to ignore it.