Two things. Copying and pasting from an editor can create a bit of an issue since all carriage returns get turned into <br /> tags. Those tags can end up between table elements that they shouldn't be causing all sorts of layout issues. The only way to get a true copy/paste is to strip out all of the carriage returns except for the ones that are actually needed.
Secondly, it is a pretty big security risk. Most of your users probably aren't going to be an issue, but you'll get the oddball that will exploit it, add javascript to steal cookies, etc.
