That's all fair and correct, but the amount written for, say, an OS, in assembly - native or IR - is quite minimal and well-controlled, usually written in assembly because of a good reason (exception handlers, for example). At that point, it's just more obvious to write in native assembly, given that you're writing architectural code, and for a different architecture, that code would be sufficiently different.

So again, I see all the value in IR for compilers. Can someone give me at least a reasonable use case for beginning to write in IR, or even using IR for optimizing cerain code paths?