With a tertiary class system, we absolutely need distinct action bars for every class combo. Here, I'll write up the basic PBI for you to give to the developers:
1. Users will have new Action Bars for all unique class combinations
2. Action bars will no longer be tied to the primary class
3. Example: When a user switches from Mage/Priest to Mage/rogue (due to the 3-class system), the user will be able to configure a specific action bar for this secondary Mage combination, while maintain the Action bar for any other unique mage/x combination
That should be enough to get the idea across from the developers. Look, you've made some progress with the tertiary class system, by adding the 3rd equipment slot, but the work isn't done yet. To complete it, you need the fresh action bars for every combination, as currently I can't easily switch between a fixed primary class and a rotating second class. Reconfiguring the action bar for every secondary change is too cumbersome and prohibits me from using the tertiary system to the fullest extent possible.
The business case for adding this functionality would be a potential for increased diamond sales due to the ability to effectively rotate the secondary classes with a preferred primary class.