Valve diameter, lift, and overlap are all contributing factors to the volumetric efficiency of an engine. If you can vary the relationship between the intake and exhaust valves you can broaden the power band and still have a smooth, clean idle. Variable valve timing is a great advance — as long as we can keep it working.
A pattern failure may indeed point you to a bad component, but when the part is expensive you want to be very sure it’s the culprit before you replace it.
A no-start on a good customer’s E46 looked like it was due to a fuel-supply problem, but automatically installing a pump would have been a dereliction of diagnostic duty.