What's the use of the average voltage in rectifiers?
Vrms is a quantity which only makes sense in the heating equivalent in a resistive load. Very few rectifier applications that I have seen have a rectifier which drives a resistive load for any heating application.  

There used to be true RMS voltmeters which actually would apply the measured voltage to a resistor and would apply a DC voltage (of course using buffer amplifiers in each case) to an identical resistor which was controlled servo fashion to cause a match in the temperature rise in both resistors, and then the DC Voltage measured would be equivalent to the AC voltage (I have one of these). In this way the measured AC waveform could literally be arbitrary in shape. There are true RMS converters that do this without heaters, and add cost to a multimeter. 

So I think your question is in relation to a average reading, RMS scaled readout of an AC voltage multimeter. This is a very cheap way to accomplish the RMS reading, but is only good for sinusoidal measured input, since the area under a half-sine between zero crossings is 2/&pi; &times; peak and must be scaled to the .707 &times; peak. Any other wave shape than half wave sine output will not correspond to the 2/&pi;. 

BTW the average of a voltage is the DC voltage at the output of a lowpass filter and this is how the rectified voltage pulses are converted into a DC voltage and scaled, in the case of a sinusoid, by the ratio of 1/22 : 2/&pi; for a cheap way to read an AC voltage as long as it is sinusoidal.