For instance, in the notes for Sands of Fate, the author mentions that he has written the secret door checks to give a bonus to PCs who have true sight
Which is the kind of thing you'd expect to see in a patch/update for the module, so that's a bug fix if anything, certainly not cheating. You weren't deliberately changing the game to give yourself an unfair advantage.
And that's certainly the way that I think about it; it's just an improvement or refinement that the author would have made himself if he had the time. (Even though it
does give the PC an advantage, albeit a minor one.) However, you can see that that sort of thinking can get slippery. Who's to say that what other things that I might conveniently think of as improvements that the author would have obviously included if he had the time? ;-)
[...] Sure I could just keep hitting the quick save key, but yeah there goes that precious immersion right out the window, when I keep having to worry about losing my progress.
I suppose I question whether magically having the ability to always respawn and continuously wear down an enemy in that manner is less immersion breaking. To me it would be more immersion breaking.
I think that icywind1980 was implying that playing while having to keep in mind to hit the save key every few minutes or whatever is what was immersion-breaking. That is, the real player has to remain sufficiently conscious of the real world to keep briefly interrupting his play to do something specific to running a program on his computer. And, it's something that the toon knows nothing about and that isn't part of the story.
Whether or not the respawn is more or less immersion breaking than a reload depends a bit on the story, but I would generally think that the respawn is less immersion breaking. And, modules with well-integrated respawns (like early in Aielund where the respawning toon meets Salinder) would likely increase immersion. And, even a standard temple respawn can fit in well with most magical worlds, at least the toon would know that he died and was brought back. There really isn't much of an in-story rationale for reloading a saved game, unless there is some sort of time-travel theme in the module.