Seeking noise is less important. I don't suppose it causes them to run at lower RPM?
No, it just changes how hard they accelerate/decelerate the heads during seeks. So I take it the noise you're getting is mainly whine/vibration? That may be a manufacturing defect (e.g. slightly off-balance platters).
Can you trust it to be available and effective on any drive model from any manufacturer?
I believe it is optional (and ignored on drives that don't support it).