[adgjlsfhk1]

> The first, and most important reason, is one needs to know the guarantees of every operation in order to design numerical algorithms that meet some guarantee

sure, but a 1 ulp guarantee works just as well here while being substantially easier to provide.

> And the higher level libs, like BLAS, are built on specific compilers that provide guarantees

Sure, but Blas doesn't provide any accuracy guarantees so it being built on components that sort of do has pretty minimal value for it. For basically any real application, the error you experience is error from the composition of intrinsics, not the composed error of those intrinsic themselves, and that remains true even if those intrinsics have 10 ULP error or 0.5 ULP error