If Littlefinger would've shown up early with the Knights of the Vale, and Ramsay knew they were there, he never would've left the damn castle, and it would've taken them months, if not years, to retake Winterfell, plus cost them a shit ton of men. People keep saying that Sansa is dumb for not telling Jon about Littlefinger coming to help, but it was actually a brilliant tactical move. It got Ramsay and his entire army outside of the castle walls because Ramsay didn't think there was any real threat. Plus, Sansa gets the added bonus of having thousands of wildlings killed in the process (you know, the north's enemy for centuries). Just because Jon befriended the Wildlings, doesn't mean Sansa did. People who think Sansa is stupid need to get a fucking clue. The only reason the siege on her old home worked was because of her intelligence.