> You're throwing away the mechanical energy.

No you are not. The waste gas vented from the system is very cold, using it as A/C costs nothing.

Although if you are sitting in traffic you are not moving, and have no waste gas.

The system overall may be inefficient, but using the cold exhaust from the engine as A/C doesn't make it any more inefficient.

In fact, if the air conditioner coils are between the air tank and the engine, the air conditioner could make it more efficient, if you don't have some other kind of regenerator-in-a-dewar trick going on.