Everyone says that incident metering is clearly better than reflective metering because the first one measures the real light on the subject, and not that reflected by its (which depends on its reflection coefficient, which the camera does not know and just supposes it is that of a grey 18% object).
Well, let's follow this reasoning. You have a subject and use reflective metering (through the camera light meter) with Spot Metering on it. Well, why should we be interested at the light falling on the subject?
What is important is the subject being well-exposed to the sensor. Not the light falling on it. Otherwise I wouldn't have chosen the Spot Metering pointed at the Subject. The light the subject sends to the sensor must fall at the center of the sensor dynamic range. Not the light falling on the subject. If I want all the scene properly expose, I'll use reflective metering with evaluative metering mode.
I've read that some sensors have a calibration constant of 250 lux seconds. This means that the light coming to the sensor (reflected by the subject) during the shutter time should be around this value to generate a well exposed image. Why should I consider the light falling on the subject?