Heating just your sump long term will most definitely accelerate corrosion in other parts of the engine. This warning is shared by sump heater manufacturers and engine manufacturers alike. I know of a case here where an owner heated their engine bay with 300W of heat continuously year round (at $709 per year in energy usage). Twice he had significant corrosion discovered inside the propeller hub which was always attributed to excessive washing and somehow water was magically getting into the hub. It turns that the root cause was just combustion moisture already inside the engine. If one part of the engine is kept warm, then moisture will not settle out there, but in parts of the engine the temperature is less and the air can no longer hold the moisture, it will condense on those surfaces. In this case the engine had a blanket over it so the temperature was quite uniform, however the aluminum propellers are massive heat sinks ensuring the hub is at ambient temperature, so this is where the condensation occurred and subsequent corrosion. I verified everything with with a IR camera, and sure enough the hub was pretty much at ambient temperature. This owner is now using a dehydrator instead.
About 10 minutes after I shut my engine down I install my dehydrator (it gives me time to not burn myself of the exhaust when installing the crankcase vent plug). The crank case humidity is about 80% at this point. The dehydrator over the next few hours will bring it down to 10% (I stated in a previous post that the humidity reduces at about 2% per hour but it is actually closer to 5% - higher flow pumps will be even faster of course).
Once you have the humidity down, you can heat the sump all you like, because with such low humidity there will be no condensation, and if any moisture is released from the sump by heating, it will just get captured by the the air recirculating through the desiccant. I have a two 100W Reiff sump heaters attached to my engine, in fact it is plugged in right now on a timer and will kick on about 2.5hrs before I depart tomorrow morning. Even on a freezing (0°C ) morning I will get to the aircraft and my oil temperature will be at 100degF and the engine will be slightly warm to touch (I keep plugs in the engine air inlets to stop circulation). I'll fire up, taxi out and take off without having to chug away waiting for the engine to get to temperature.
Details on my dehydrator may be in the link below.
http://www.vansairforce.com/community/showthread.php?t=157922
So to answer the OP's question, engine preheaters are great if used only when necessary before start and in conjunction with dehydrators, at least based on my analysis and experience.
I hope this is of help.
Tom.
RV-7