it isn't that you need them, I fly them as another flight control! there are things that are not physically possible without full control of them, as daveopam mentioned. doesn't matter how your plane is set up, it will drop faster then the same plane with full spoilerons on. If I want to drop vertical, pull out at the last second, and land, there is nothing I can do in a regular setup to make this possible, but with full spoilerons/flaperons, it's easy.

how about REALLY short takeoff distances. again, doesn't matter how your plane is set up, unless you can drop your ailerons way down you still won't be off the ground nearly as fast as the guy who sets this up.