Currently, you hold shield and then attack to shield-charge. If this got removed, a lot of people would be irked over the loss of an advanced technique. (Shield-charge is when you start charging a weapon without performing a regular attack first, letting you start the charge faster.)
In addition, double-tapping only lets you dash easily in four direction and only at all in eight. You may want to dash more precisely than that, hence a button.
That being said, I don't see why dash and bash can't be the same button, modified by whether or not you're currently shielding.