From my perspective of implementing algorithms it matters because if I want to follow the spec correctly, I have to deal with them. Even if you don't want to use them, you pay about 10% for pretty much any function that needs special NaN handling (which is most of them). Also, pre AVX-512, these checks make vectorized algorithms way harder to write efficiently.
I agree that if you are writing a library to be used by others, you cannot know how they will choose to configure their floating-point environment, so you are forced to handle all the possible cases in your code.
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From my perspective of implementing algorithms it matters because if I want to follow the spec correctly, I have to deal with them. Even if you don't want to use them, you pay about 10% for pretty much any function that needs special NaN handling (which is most of them). Also, pre AVX-512, these checks make vectorized algorithms way harder to write efficiently.
I agree that if you are writing a library to be used by others, you cannot know how they will choose to configure their floating-point environment, so you are forced to handle all the possible cases in your code.