Robb hasn't made the best of decisions but I don't think holding Rickard Karstark captive would have done much to 'control' his army. Sure he could have kept the army intact for just a bit longer but those Karstark men would hardly be loyal to him. They would probably seek out the first opportunity to rescue their lord and backstab him.

Robb lost control of half of his army by losing control of Rickard Karstark. It was too little too late to salvage the situation. I dont think he had any choice but to cut them loose.