I think... and maybe this is me just trying to rationalize the stupidity of some people, that when someone leaves a review like "Update soon!" They are trying to be motivating. Now, granted, its a pretty shitty way to go about it and it's freakin' maddening as an author, but I think they mean well...
Of course, how much longer does it really take to type up something a little more intelligent? Not long at all. Even something as simple as "I really enjoyed your story" is better than an "Update soon".
And of course, if they want to beg and plead for an update on a FINISHED STORY(!!!) well, everyone's free to torture themselves in whatever way they like, so hey! Enjoy to pain of waiting, cuz it's definitely going to last!