If I recall, it's significantly buggy if you merely enable this.  Highlighting doesn't work with incremental search without additional code changes, it doesn't go away when you close the Find bar, etc.  There's also the question of what should happen if the user edits the text in a way that adds/removes matches -- I'm not sure whether this patch addresses that.

I did a little work over the break playing with Find highlighting, so I'm happy to review these code changes when I'm back on Friday...