The D3 definitely needs to know pad thickness as it uses that to calculate when the pads are worn down and hence control the light. If you change the pads without telling it the thickness of the new ones one of two things happens:
1, it still thinks it has old, worn pads and puts the warning light on before the new pads need it
2, it has no idea what pads it has so doesn't both with the warning light at all. The first sign you get of worn pads is it wearing through to the metal and destroying the discs! That is assuming, of course, that you rely on the warnings to tell you when the pads are worn.
It uses the servo in the motor to determine how much the piston moves by when it applies or releases the parking brake, but it doesn't know the absolute position of the piston, hence why you have to tell it the pad thickness so it can calculate the change from the piston movement.
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