2 pole RC instead of LC filter in Buck
Inductors and capacitors give and take reactive power. This means that they charge and discharge power.

Resistors consume real power, the only way of getting that real power back is by converting the dissipated heat back into electricity, which is not realistic. 


Inductors stores its energy in its magnetic field. 
 The energy in the magnetic field comes back in the form of electricity without any extra fuzz.
Capacitors stores its energy in its electric field.
 The energy in the electric field comes back in the form of electricity without any extra fuzz.
Resistors waste energy in itself by heating up.
 The energy in the resistor in the form of heat is difficult to turn back into electricity. Most of the heat will heat the air around it through convection. Using a resistor is simply too inefficient. 


So if you want a filter that is efficient, then you don't want heat, heat is the result of energy being wasted. Instead of wasting energy, it can instead go to the load, which is your ultimate goal. 



Edit:

Compare two LP filters. One RC and one LC based. The load will be entirely resistive.



Link to simulation.


Upper graph's are the power consumed by each component in the LC based filter.
Bottom graph's are the power consumed by each component in the RC based filter.


The L and C graph's are entirely reactive while the R graph's are entirely real. 

Notice how the average power in the L and C in the LC filter is 0. No energy is lost in the form of heat. The CLK is a square wave that goes from 0 to 5 V and has a duty cycle of 50%. The 1 kΩ load gets all the energy which results in 2.5 V across the load.

Notice how the average power in the R in the RC filter is not 0. It is wasting it in the form of heat. And look at the 1 kΩ load, it is not at 2.5 V, it is at 2.3 V. The 0.2 V lost is due to the R in the RC filter, \$P=\frac{V^2}{R}\$.

Something else that might be interesting to see is that the amplitude of the RC filter's power is much larger than the ones for LC, so the LC filter is much "kinder" to its power supply. It causes less noise than the RC filter.