Why would you want a SMPS to be PWM under light load?
Your switcher has an energy storage device (an inductor) that you are charging and discharging on every cycle.  It has to be large enough to handle your heaviest load at your PWM frequency; each cycle you are storing a small packet of energy and passing it to the load.
When your load becomes light, if you don't change frequency you are switching much more often than is necessary and passing along very small packets of energy compared to the amount of energy that could be stored in your inductor.  By reducing the frequency, you are utilizing the full storage capacity of your inductor but not experiencing the inefficiency of switching as often, so efficiency goes up.  So in applications where the load can be light and doesn't change suddenly, it might be a choice.  However, if you have a rapidly varying load or variation in input voltage, the low frequency switching combined with the control loop that gets you in and out of PWM mode will lead to variations when you suddenly pick up or lose a heavy load or drop or increase your input.