He'd be the controlling member of a House Bolton, married to Sansa, and presumably become Warden of the North and Lord of Winterfell. Besides, by hitching his wagon to Stannis, he somewhat purges himself of the history that House Bolton has -- you know, the Red Wedding, which Ramsay himself had nothing to do with. Oh, and most importantly, he removes a challenger to his titles in Walda's unborn child.
I mean, had he gone this route and treated Sansa better, this would've been a fairly brilliant play.