I live in California, near the coast. So, yeah, I've had some cars that were pretty well rotted away but it's not near as bad as in areas where they salt the roads. So, no, I haven't snapped or rounded off any bleeder screws or caliper mounting bolts. Hell I've been lucky and haven't even stripped one of those stupid hex retaining bolts on the Bimmer.
If it's my car, I'd want to make sure it doesn't get to the point of frozen fasteners (make sure the rubber boots are all in place, that you've got anti-seize in the right places, that you're regularly flushing the brake fluid, greasing the sliding pins if applicable, etc).
That said, yeah, I know brake fluid isn't particularly flammable, but I'd still rather not deal with hot fluid in a confined space.
Knew a few mechanics from So-cal, they all said "I had no clue! I never had to fight like this just to remove a bolt before" (except crank pulley bolts)
Ya, preventing it in the future is a good thing and somewhere I think I mentioned using antiseize and lube, but since he hasn't maintained it before, and it is used (atleast somewhat) we have to keep the possibility in mind that some type of oxidation has already occurred.
Ok, no dispute hot brake fluid can be bad.
Having said that, the I end up using big hammers, oxy-acetylene, impacts, prybars, extractors, and air hammers on a daily basis to fight corrosion. It is an unfortunate occurrence, and a mutli-billion dollar a year industry. It is worse for my industry since vehicles travel the whole country and get put away wet, but it is a fact of life for many cars too. Nearly everything I work on gives me some kind of grief due to corrosion, even aluminum and stainless. Hell even steel into plastic. Whether it is seized fittings, corroded wires, broken/seized/rounded bolts, rusted out components (causing something to need to be replaced in the first place).
I owe my livelihood to corrosion, yet it is pretty much my single cause of stress/headache/injury.

Could not imagine even having a job if it wasn't for that.