One likely problem with that reaction is that it isn't easy to reverse. You'd end up with a lot of sodium borate (jagshemash!) that would be difficult to reprocess back into sodium borohydride. Ideally you don't want by products from you fuel cell other than water. Anything else is dead weight you are carrying around.
Production of the borohydride in the first place requires a considerable amount of energy. Overall, it's cheaper to use liquified hydrogen. However, that's also extremely dangerous.