The benefit of branching is that strings can be re-worded, deleted or added at one branch without affecting an other one. Of course, the trade-off is that the maintainers have to maintain multiple versions of their language.
In order not to have to translate every string twice (once at 2.0, then at 2.1), there is a merge tool available for the language pack maintainers at the Stage page. The merge tool can copy translated strings from one branch to another. Therefore, I would suggest that translators continue working on 2.0 branches and they just merge their work to 2.1 from time to time. The opposite way is possible, of course - work in 2.1dev branch and merge back to 2.0.
To have a report of the differences between branches, a new AMOS Diff tool is going to be written (hopefully soon). After that, it should be easy to detect all places that require special attention as the translation for one branch (eg 2.0) is different from the another branch (eg 2.1).
Let us see how it works. Enjoy!