PWM wouldn't really make a good carrier wave by itself. being a pulse train means it has a lot of harmonic content. the harmonic content is highly variable because of the changes in the pulse width. however, in many modern AM transmitters, PWM is used as a method of creating AM modulation. what is done to do this is the AM transmitter generates a carrier wave, which goes into the final amplifier, and the supply voltage for the final amplifier is modulated by the PWM, and filtered, so that there are audio variations in the supply voltage of the final amplifier. the RF out is at the carrier frequency, modulated by audio. the PWM pulse train is filtered out and does not appear on the transmitter's output. this is much more efficient than a traditional AM modulator, which requires a large (audio power that is equal to the transmitter power) amplifier and very large modulation transformer.
i've actually seen one in use, and the original transmitter consisted of four cabinets, exciter, amplifier, modulator, and power supply. the one that replaced it used PWM, and consisted of three cabinets, exciter, PWM supply/modulator, and amplifier.