Hi,
Yes you guys are getting to the main point here and that is that the most important role of the inductor is energy storage, and that energy storage is what allows a buck converter to be called a "true" power converter, whereas a resistive divider (transistor or not in there) is not.
A true power converter actually converts energy from one form to another and that's the mechanism that makes it work. It takes a higher voltage and lower current and converts it to a lower voltage and higher current, similar to how a transformer transforms power. For a 100 percent theoretical system, the power in equals the power out but what changed was the voltage and current, so that makes it a true power converter.
Take out the inductor though and what we see is a pulse with modulator (you can add resistance to reduce current) but that only means that somewhere there has to be resistance eating up power. It is true that the average current will go down, even the current for each pulse, but the current pulse will still be high enough where any resistance (necessary resistance to limit current peaks) eats up power.
If you analyze this you find that as you lower the resistance to try to reduce losses, the current peak goes up so you end up loosing a lot of power anyway.
So without the inductor your efficiency drops quite a bit except in the case where the output voltage is almost the same as the input voltage. When the output voltage is almost the same as the input voltage then the loss in any resistance starts to equal the loss in a real life buck converter where it can not be exactly 100 percent efficient. With a typical efficiency of 80 percent, this means somewhat small but not insignificant differences between input and output voltages. For example, if you want to drop only 1 volt with an input of 10 volts and output of 9 volts you may never see a buck that can equal the efficiency of just a series resistor, depending on the current requirement. But for 10 volts in and 3.3 volts out for example you'd never get the efficiency anywhere near as good with anything but a true buck circuit even at 80 percent efficiency.
We could look at more specific examples.