Symbol completion is useful, but can get in the way quite easily. When it has useful information to convey, you can live with that, but when it pops up "no matching completion found..." it is simply noise. In certain cases it can be very detrimental to typing performance.
Example: typing multiline comment text, without auto wrap, it can be difficult to start a new line, since the return key becomes effectively disabled when the previous word doesn't match a symbol.
Not displaying, or removing the popup when no match is found should improve usability. I can turn the feature off and use kate auto completion, but that is not as powerful and kscope's completion.