Basically, what this boils down to, as I see it, is paying extra to fix problems with content we've either already paid for in previous expansions, or should expect to be fixed within the scope of our subscriptions.
Bug fixes done as part of booster development are rolled out to everyone whether they buy the booster or not. In addition, there is also feature "spillover". For example: smooth boat movement will be rolled out to all players in the same client patch, and boats will move smoothly on everyone's screen, whether or not they purchase the booster. You won't be able to use the new ships if you don't buy it, but the classic ships will continue to be available to everyone and they will benefit from smooth motion and bug fixes. So, the entire game benefits, and those who purchase the booster benefit more.
With bug fix work, there is a sort of economy of scale as well. If the team spends a significant portion of its time intensely focused on a few systems, then there is synergy with both the development and testing cycles, so more work gets done with the same number of man-hours.