My disagreement is, I suppose, more quantitative that qualitative.  Yes, the vent tube is low compared to the filler tube, so air will rise in the vent tube, be heated and then flow into the engine, presumably purging the "bad stuff" out the filler door.  This convective action takes place only while the engine is hot, so it dissipates rather quickly - as Brent proposes, the most effective time is soon after shutting off the engine.  But the effectiveness depends on the internal design of the engine, which most certainly was not based on this type of venting after shutdown.  On many (most?) Continentals, the vent tube is connected directly to the filler tube, making any purging of the crankcase pretty much impossible.  Lycoming engines have the vent at the rear and the filler somewhere else, giving at least a possibility that some purging could take place.  I guess I'm just not convinced that the benefit is worth the potential risk.  As anecdotal evidence, I received one private reply from a practitioner that said he forgot to put the cap back on "only" twice.

Gary Casey