I can think of one useful property of subnormal numbers off the top of my head. If subnormal processing is enabled, then for all finite values of `a` and `b`, `a != b` if and only if `a - b != 0`. But if subnormals are flushed to zero, then two tiny normal distinct values `a` and `b` would have a subnormal difference that is flushed to zero.
Isn't that just a scale issue that exists with or without subnormals? If a and b are closer to zero than the smallest representable number, a and b compare as the same. With subnormals your smallest possible number is smaller than without, but it's still the same issue.