It's not a matter of expect to be written to disk or not, it's the performance tradeoff versus data security should it not be written to disk. An MTA must make sure that it's committed to disk because it's irretrievable if it's not. A word processor autosave, not so much. Non-error log files, same thing.
My machine has certainly been writing things to disk all while I'm reading lwn here (logs, browser cache. If I were at home, it could be bittorrent, etc). My life wouldn't be any poorer if the system were to crash right now and none of that were recoverable.