Should be a BIOS feature on every motherboard in my opinion. The higher quality ones should have this option for any PWM fan already.
The difference between a low speed curve on a silent fan (e.g., Noctua's 120mm design spins at a low speed of 300RPM) and off is hardly audible either, especially inside of a case, so it's a niche really.
If there are awesomely silent fans out there (such as a Noctua especially with the LN adapter installed) then why bother with motherboard-based semi-passive controls? At very low speeds the high-end PWM Noctuas (and probably some other premium LN fans) are inaudible anyway - regular PWM controls work perfectly and semi-passive is unnecessary and in some cases undesirable.
The heat sink should be more than enough for the CPU at idle, and a single exhaust fan circulating air through the chassis should be all that is needed. The CPU fan should
shut off when it's not in use just like a GPU or PSU, and they often don't. It increases lifespan for the fan (even fluid bearings) and prevents any potential noise (again noting that high quality fans are typically inaudible at their lowest PWM function). Good design is good design, what can I say?
But yes, my point was that the 0 RPM function is rather moot and should be a motherboard feature.
To be fair, I've actually had three motherboards with this feature built in. Though, since Arctic can know the typical amount of noise they generate at various RPMs and calibrate the fans with that in mind, I have the general feeling that this can regulate itself better than the motherboard could do it.
Au contrair. Unless you build a PWM controller that uses infrared heat signatures the motherboard will always do it best.