It makes you look stupid. Extending this courtesy might have had a tinge of validity before radio, when few people heard public officials speak, but in the age of 24-hour TV news, fiddling with people's words to make them look superior to you and me is irresponsible. And then there are cases where an editor might want to make people look stupid, or at least colorful. But it does mean that a reader should expect the writer to stop and the speaker to take over once an open-quote mark appears. We should have a good reason for editing a quote, and we should have an equally good reason for using one unedited if it might be embarrassing.
Quotes draw readers in, giving human warmth and an approachable tone to abstract news.
They crystallize a story's focal point. As transitional devices, they carry a story forward. Both Associated Press and United Press International style guides acknowledge circumstances in which editing a quote is not just appropriate but recommended. In fact, UPI strongly advises writers to remember the difference between spoken and printed language. News writing isn't a transcript of reality.