The playbooks are so diverse and independent that it can be hard to see how they might consistently work together, they are mostly leaders in their own right, and infused with personal responsibilities right off the bat. The glue, I think, is oaths. Oaths were terribly important in history, binding promises that dictated lives and were the difference between honor and dishonor, freedom and outlawry. Oath breakers were considered deeply untrustworthy, and people would hesitate or refuse to deal with them. Sagas of the Icelanders has a wonderful section on honor which details the effects of having it and not having it, and I would encourage everyone to read it.
Though Oaths have a mechanical effect and function like bonds, they are in fact promises to another character that players are rewarded for keeping to. Though there is no mechanical disadvantage to breaking an oath, the fictional implications are enormous.
If Dahved the War Herald has sworn to return Alej of Tyrene the Outlaw Heir to his throne, and Alej has sworn to lead Dahved to glory, it gives the characters an agenda to pursue together, external threats to deal with, and internal strife to overcome. The mechanical benefit of pursuing the fulfillment of your oaths is experience, the narrative benefit is the advancement of the character's goals, and abandoning or shirking those oaths will generate all kinds of fascinating drama.