There's a lot more than oil and water in the salad dressing in the crankcase.
 100LL contains tetraethyl-lead which, when burned, produces lead vapor which
deposits on everything it touches.  Some of it even gets on the valve seats
where it does some good, while the rest goes to all sorts of places we don't
want it to.  Over time, this would build up to unacceptable thicknesses.  
Something has to be done.

Enter scavenging additives.  These are combustible additives that contain
REALLY nasty, corrosive, and sometimes carcinogenic chemicals that clean out
the excess lead.  They are also really bad pollutants and are the unspoken
environmental reason for banning leaded fuel.

Traces of everything the engine burns end up in the crankcase, especially
with the sloppy piston clearances of air cooled engines not using TBC's.  So
along with the oil, water, ash, shellac, carbon, lead, etc., there are trace
amounts of these corrosives.

Water by itself isn't that corrosive.  Where its power comes from (if we want
to call it that...) is in dissolving almost anything ionic into individual
positive and negative ions and creating a corrosive solution.

Keeping the engine warm all the time is asking for corrosion;  warming it up
just before starting it makes sense, since everything will be much closer to
operating clearances.  Plug it in during the 30 to 60 minutes that it takes
to load up and get the weather and it wouldn't have much time to cause
corrosion.  Like the mixture control, engine heating can be not-used,
misused, and abused.  It's all up to the operator.

M2c,
Eric Ahlstrom