There are reasons why you would want to keep ultrasonic frequencies out of your DAW. A contrived example is where high frequency content can affect compressor behavior, causing audible frequencies to be compressed in a different way than if the ultrasonic content wasn't present.
I say contrived because it is unlikely that you will have that much ultrasonic energy present, but the point is that just because you can't
hear it, doesn't mean it
can't interfere.
Another example is plugins like tube emulators that have non-linear transfer functions and hence
will create intermodulation distortion when ultrasonic content is present.
Maybe the actual effect of ultrasonic frequencies on the audible range is minimal in practice, but people have no problem arguing about 32-bit vs 64-bit floating point differences. The difference caused by ultrasonic frequencies on the audible range is probably significantly higher that the differences between 32-bit and 64-bit processing.