Clever for reducing the component count in the car, not clever for performance, if you're trying to heat the cabin. Whether the heating is done in a dedicated heater element or in the windings of the motor, it is still a resistance heater.The Model 3 uses the rear motor to generate heat - up to about 4kW apparently. It is a very clever system.
Furthermore, it is considerably less efficient than using an air space resistance heater due to the huge thermal masses and delays involved.
If your resistance heater is just coils of nichrome wire in the air flow coming into the cabin then when you turn the heater on the coils of wire have extremely low thermal mass, heat up almost instantly and put out hot air immediately. That's basically the same as a fan heater.
If you heat the motor windings on purpose there is a large thermal mass of motor that has to be heated before the coolant flowing through it starts to warm, that coolant loop itself has considerable thermal mass and heat transfer rate is limited by the rate the coolant is pumped around the system. You then have to pass that coolant through a heater matrix if you want to use it to put warm air into the cabin. To get any sort of effective heating you need a coolant temperature of at least 50-60C.
So as a means of heating the air in a cabin it's really poor. This is one reason the heater in my Ion is so crap - it does more or less the above except swap the motor for a PTC heater. It has a PTC water heater that heats coolant in a coolant loop that is pumped through an old fashioned ICE heater matrix. It takes ages to heat up (5-10 minutes) and cool down, and there are a lot of heat losses from the heater itself (mounted under the car) the pipes etc. In short its very poor compared to an air space resistance heater let alone a heat pump.
Where the waste heat from the motor could be usefully used would be to warm the battery however. The heat is already available in liquid format ready to be pumped around the coolant channels in the battery, and the target temperature you want is much lower than that required to heat cabin air effectively. The Model 3 does in fact use excess motor heat to heat the battery.
It's still not as effective as a dedicated PTC heater (as used in the Model S / X) for pre-heating however, due to much larger than necessary thermal masses and the fact that some of that energy is being radiated from the motor into the outside world. So not ideal from a pre-heating perspective (deliberately heating a stationary motor) but a good idea for recovering waste heat once the motor is warmed up naturally from driving.