"I thought if the compressor takes a dump you need to replace the receiever dryer too?"
Anytime your system is opened up you need to replace the R/D.

"vacuum to the system and suck up everything,"
You really can't do that because the R/D has dessicant in it... to get and keep the very last moisture possible in the system. You can not get the moisture out of the oil load and dessicant in the R/D by vacuuming... .thus the suggestion that it must be replaced instead of ( for instance ) just vacuuming it to a certain spec...

"Suck it all out then and refull the system."
Most people do not have the expensive machine ( or a homemade version either ) to hold the R-12 while it is out. Would be less than frugal to loose the first charge and have to put in new R-12 when the standard operating procedure works fine.

I don't know what that AC instructor was thinking.... or what books he was consulting.... if you still know him you might ask where he has seen that concept recommended in writing... all the features which I have described here I can show you the sources from Multiple references...

"That should get most of it out."
The effects in terms of degraded system and potential for acid production is such that we are shooting for ALL of it out... as far as our procedures and equipment will accomplish....

" it gets to stay in the R/D and should not be a problem."
The R/D has a rather small capacity... is a one way function in terms of contamination... this is why the rule is to replace.... and replace at the last possible moment with regards to the system being clean...the R/D hopefully takes up the slack on anything left and holds it safely.