Why  use a resistor when blocking DC?
Amongst other things the resistor serves to provide "DC restoration".
 ie it gives the output AC waveform something to vary relative to. 

Imagine you were going to use a comparator to provide zero crossing information about the AC signal. Without the resistor the question is "zero relative to what?". In the real world an isolated output may acquire or lose charge. The drive capacitor may leak, light may selectively remove or add electrons to the output circuit, the wind from the mountains may have electrons stripped off its molecules and it may then deposit positive charge on the output circuit - which unlikely sounding circumstance is what happens with the French Mistral and various other winds.  The resistor provides a reference path. Anything big enough to move the output relative to ground must drive the resistor and so have finite / real power.

As you say - the RC form a low pass filter.