If Stannis loses the ice battle I'm going to be rather disappointed; from a narrative point of view it won't add anything to the story imo. Everything has broken right for him. He found the traitors/got rid of them thanks to Jon's letter, he knows when Bolton's men will arrive for the most part, the Iron Bank plans to fully finance his claim, he controls the high ground, and he holds a key to the North's current loyalty hostage (Theon). All this while the Lannisters have to potentially deal with the Tyrells and "Aegon;" if Stannis wins the Winterfell battle he will essentially win the North.
If Stannis can secure Winterfell, it sets the stage for the return of a Stark, probably Rickon. It also puts a wrench in Littlefinger's plans for Sansa, potentially at least. Will Littlefinger risk sending the Eyrie host off to siege Winterfell now that winter is here? And when Sansa is revealed, how will the Northern lords respond?
Holding the North might not matter much at all when/if the Wall falls though