The sensors work fine in my experience, as long as the four pads on either axle wear at the same rate. If they don't, you may get metal on metal before the warning light.

On our X3, I noticed that one right rear pad is wearing faster than the other, it is thin, and the brake pad light isn't on yet. I will change the pads soon and not wait for the light. I was in there to change the rear springs out, and saw it; the vehicle is 7 years old on original brakes with over 100,000 km so not surprising that they are worn, and it is likely that cleaning up the caliper slides and applying high temperature grease to the pad ears will correct the wear imbalance.

To the OP: if you trust your mechanic, then yes, it is likely that you need brake pads even though the light isn't on yet. Don't turn the rotors on a lathe, they aren't very thick. If they have worn thin or the surface finish isn't good, it is better to just replace them.

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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