Just to clarify my thinking. . . a block heater is not only desirable for starting ease. . . . it also reduces the strain of weak lubrication at startup. . . and subsequent wear on engine and starter parts.
While I'm quite certain that I "can" start my Massey at -10 degrees without using the block heater . . . Why would I want to? The coldest temp I plowed in this winter so far was -9 degrees plus wind chill in an open (non-cab) unit.
My tractor sits outside in cold weather. . . It always starts. . . but. . . helping the tractor start as easily as possible and with the least wear possible is just the most reasonable action unless you have inside storage for it.
Since you seem to be asking, my reason is, I don't believe my engine is of such poor quality, that starting it cold occasionally, is going to do serious harm.
I also don't care to be ripped off by MF, paying 5x the going rate, for a block heater I personally don't need.
So, do you also have a heater on your tractors transmission fluid? They greatly reduce hydraulic pump wear, and engine wear, due to excessive warmup periods in cold weather.
How about block heaters, and transmission warmers on your car engines? Doesn't cold starting wear them too?
Do you have pre-oilers on your vehicles, and equipment? They greatly reduce engine wear on start up. Even more than a block heater could ever do.
So, as you can see, it's all a calculated risk, which can only be avoided by not starting it at all.