It was always a marriage of convenience. Tywin’s entire strategy hinges on giving them just as much as is needed to keep them on-side and hungry for more, without giving them too much and make them independent of the Lannisters.

The North is not only a huge asset in and of itself, but a marriage alliance between House Tyrell and House Stark possibly reorients the political cast of the war:

Bloody fool, thought Tyrion. “Sweet sister,” he explained patiently, “offend Tyrell and you offend Redwyne, Tarly, Rowan, and Hightower as well, and perhaps start them wondering whether Robb Stark might not be more accommodating of their desires.”

“I will not have the rose and the direwolf in bed together,” declared Lord Tywin. (ASOS Tyrion III)