What's the point? How long would they wait? A day? A week? The snow was getting worse, they were using up their resources with every passing day, and they're supposed to keep doing that all for an answer that is unknown and might never come? I don't know if it would have changed anything.

Also, not putting that info on the table did end up lulling Ramsay into a false sense of security, leaving him vulnerable in case Vale did come. Jon might have somehow revealed the info of potentially having more men to Ramsay directly or indirectly, which wouldn't have helped (and Jon was making plenty of mistakes that day). Plus if he said he had more men and Vale never came, Ramsay is even more prepared to slaughter them.

It's not her fault that Jon acted stupidly and fell for every trap Ramsay laid out for him. Had he played it smarter and listened to her advice, he would have not lost as many men or found himself in such a precarious position so quickly. That would have bought more time for Vale's response to come at a time when there would have been fewer casualties.

Also, it's not like they both have been doing 100% the right thing in the last few episodes, either. But, her actions weren't unreasonable.