The problem with this idea is that some professions are simply more popular than others. In turn, certain professions story content would makes lots of money, while others wouldnt. In turn, Bioware would "follow the money" and make lots of content for the popular professions, and only a little for the non so popular ones. So the end results of such an idea would be the most popular professions having lots of content packs, while the not so popular professions would only have a few.
If you follow a particular track and lets say that is true, that isn't the only way it could go down.
You still could in a calendar year hit all the professions. However, the allocation and resources would be divided up based on sales. Even the least profession would get some love in the form of class stories, but theirs might not be as meaty as the more popular one. Thats one way to mitigate that cost while keeping every profession relatively happy.
Also consider that it would likely take them a calendar year just to add all 8 class professions to that planet. So they aren't out of the gate going to be able to just pick a profession and say "we're just doing this one". After all, just because there's more people in one profession doesn't mean that those people are more willing to spend on a class story over another profession.
The point is, there's a way to go round robin on the class stories and still manage the money going forward by dividing up the allocation of resources based on sales - everyone still gets class stories, but the top quality goes to the one with the most sales. As sales and tastes shift, so does the resources on an annual basis.
Seems fair to me.