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