I agree that often coils do not slowly wear out, they fail either open or shorted. But they can start to fail and provide a weak spark, if the resistance is not in spec. I've seen that on older vehicles and not these engines, so I don't know how common it is with these particular coils. These are not simply transformers (as asked above) since the primary is DC, not AC. These are induction coils, and the current in the secondary is induced by the collapse of the field in the primary. If the resistance is not to spec, you will not have the correct current flow, and therefore you will not have sufficient current flow from the secondary, resulting in a weak spark. The resistance can be out of spec simply due to the coils being too hot.

Even if it is a complete coil failure, ie no spark, that failure mode can be intermittent, as ChuckG noted, and it can be temperature dependent. You could check coils by measuring the resistance on older vehicles, not sure about these ones. If you do have an intermittent problem on a single coil, or a weak spark from a failing coil, those are the situations that can be improved by installing new coils, making the vehicle feel less sluggish.

I would just suggest that if you have a problem with ignition coils, replacing them with either new aftermarket coils or new OE coils will have the same effect. I see little benefit to aftermarket coils over properly functioning OE coils given the sophistication of the ignition and engine management systems.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue