To be more specific, the reason Quicksaves are not as trustworthy as autosaves is because the game is more likely to corrupt saves that it constantly overwrites as opposed to simply creating a fresh save file every time. Same reason why the Autosave feature is discouraged.
That shouldn't really be happening, though. Writing to disk is a function of the operating system, and if this is happening, it's a Windows (or xbox, or whatever...) problem. I'm not saying it doesn't happen, just that it should be rare.
I've never had a problem with using quicksaves (or autosaves), and have survived a few sudden crashes as a benefit of using them. I'd say that it's a good policy to keep "hard" saves, but that there's no harm in using the quicksave feature for added assurance. Just don't rely on them as your only save system.