I think it was very poorly implied that his release was the breaking of Tormund's band, which of course begs the question as to why Jon took so long in letting him out.
The one thing that does not really make all that much sense both in the books and in the show is the idea that the initial wildling attack was merely testing the remaining strength of the Watch. It sounds okay on the surface but in reality, Mance wasted his best chance to overcome the Watch but not throwing everything at the tunnel immediately when Tormund, Ygritte and the Thenns were attacking Castle Black from the south.
Support Mag the Mighty in the ice tunnel and the Wall falls.
Now, even though the Watch has suffered further casualties, he won't have this diversion and surprise for his main attack and he has also given any reinforcements the chance to arrive.