Leaving it open could induce a 'draft' from the interior up the chimney
flue. 
Certainly while motoring down the road. 
This would create turbulence in the combustion chamber and can very
likely cause the pilot/flame to be blown out. 

The combustion intake & exhaust are in the very near same place on the
outside of the van body to reduce chances of an atmospheric pressure
differential which would create an excessive draft and blow out your
fridge flame. 

It is the close proximity that allows it to remain lit during driving.

Dan Stevens
Empire State VW Camping Club
------------------------------------------------------------

From:    Keith Ovregaard <kovregaard@GMAIL.COM>
Subject: Re: WHat is this for

You got me curious about whether the drain plug should be always be in
place or not, so I looked it up in the VW manual and the Dometic
manual. Interesting that nothing was mentioned in the Dometic manual.
The camper manual only mentioned "To drain accumulated water due to
normal condensation, open the screw on the condensation drain pipe."
I would assume that in normal operation you should leave the plug in
place. I've always left mine open! Now I wonder what, if any,
performance difference there would be. It also said that the flue vent
must be kept uncovered at all times and only when you wash the camper
or when using an automatic car wash should it be covered. It makes
sense that it should be off during operation of the fridge, but why
would they say "at all times"?

Keith O