Comment 5 for bug 379820

Yes it would do harm, noincremental can take forever, on the other hand their is no gain since it would get executed as root and thus no other user would have any advantage from it.
Also I'd like to note that kbuildsycoca4 is executed upon login, so there must have been some other issue if reboot didn't fix it (probably something filesystem related, since kbuildsycoca will only re-read a file when it's timestamp changed). In addition to that, as mentioned, desktop files that are translated via language-pack-kde do not have any translation in the files themselfs, but the translation's mo files which gets loaded on application startup (every app startup that is), so kbuildsycoca4 should have nothing to do with those translations at all.