There are downsides to leaving your block heater plugged in continuously; the increased electrical costs and a shorter (number of years not hours) lifespan of the heater. A warm diesel is a much happier diesel when started. The block and head are warmer as is the oil. We had a twin Detroit powered fishing vessel that was plugged in all the time. The marina restaurant complained about the amount of unburned fuel coming out of the pipes after start up. Leaving the block heaters on all the time eliminated this issue and made everyone at the marina happier.
Randy