I would say that it is somewhat similar to using Mint's trashing. That is, sometimes you will luck into it. Sometimes it seems that all your good cards clump together by chance, and this is may be a good time to pounce on a pricy Doctor. It is analogous to that fortuitous 5 Copper hand in a Mint game. Unfortunately, since this happens by chance, the deck you were constructing might not want or be ready for a mass trashing.
Alternatively, you can work towards a Doctor cleansing "megaturn" in much the same way you would with Forge. Here you want to either draw your deck and discard the junk, or you want a strong filtering engine to go through and grab the good stuff. Then you buy a Doctor and trash all of the junk. Now, if your engine is strong enough to do that regularly you probably don't need the trashing. But if your fledgling engine just happens to get that lucky turn, that you can use that opportunity for a Doctor to vastly improve future reliability.