The main reason to use a block heater, IMO, is to help prevent damage from cold starts. The vast majority of wear on an engine happens during start-up and in the next few moments after starting, until the oil is flowing freely throughout the engine. On a cold start, this high wear period is greatly extended, since even multi-vis oil takes longer to start flowing to all the little nooks and crannies when it's cold. It's even more of a problem since clearances are reduced in very cold temperatures: the very time you need lubrication the most is the time you have it least. You will probably never notice the effectsa of a couple of cold starts, but over time it adds up to something very significant.
Yes, a block heater helps you get a deisel started when it's cold, but don't forget that it also adds to the longevity of your engine. For about $60, it's cheap insurance.
John Mc