If the North helped the Westermen, it would the alliance that nobody expected, no doubt about it. But it would be a swordbreaker for the Reach, IF it conspired that the North was going to ever send men down, no matter how long it would take. It would either force a huge escalation, or for Garland to sit the fuck down. The Reach would need to have an ally of their own, and who knows, we haven't seen the Baratheon or Martell response too, nor the fact that the Iron Islanders are looking to intensify their Northern raiding. 

Don't think Alerie hasn't got her own plans. She knows the Crakehalls are going to be angry, of course- but she knows it is bold moves, in the light, that are going to hide her actions, even behind her brother's back. Let's just say, the Tyrells do not just have numbers or money on their side, they happen to have a Yunkish slaver...