This is exactly what I started doing yesterday, it's those "grace periods" and "warm-up time" factors that made me realize two things: A) It's not a horrible idea to "jumpstart" certain high-temp cookjobs with a high-temperature fuel like charcoal and then proceed to use the intended fuel, but it's still relatively inefficient, and B) You get the best bang for your buck for doing larger and uninterrupted cookjobs, as long as you maintain the fuel supply until the job is done.
I can see how someone would do this in a situation where fuel sources are scarce, i.e. no ready access to peat or coal and maybe they are far away from trees and/or have a very long sapling growth rate for some reason, so I bet some hardcore desert dweller might benefit from babysitting their campfires in this way when every log is precious.
Considering this is a game where there are always dozens of chores to do if you're playing it right, I definitely don't see a situation where I would do this myself even in the very early game, but it does give interesting insight into the cooking mechanics.