When developing any sort of application that will eventually be used by non-developers the above phrase is a very common one to hear...but here is a thought: if you "slap" a hook onto a door, will people other than you know how to open it? 

As much as careful consideration and thought is taken into designing and implementing behind the scenes technologies, the front end is a fairly important layer technology that should also not be underestimated nor overlooked. Of course, if your application doesn't have a front end then you have nothing to worry about.

Now imagine sitting for dinner at a nice restaurant, with excellent service time, friendly waiters, but your appetizers, entree and dessert all come on the same plate. The whole experience is ruined and it won't matter that your steak is grilled to perfection. Thats what "slapping" a UI on it looks like.