Not sure if I'm right as I never watched the entire list of caster weapons yet.
But, if logic applies, if you remeber, TBC, Vanilla content had a nice mix of 1h+OH as kinda' BiS but also a variant in the form of a Staff. In the first Tier of content in WotlK (Naxx25 etc) the best combo was by far 1h+oh for every caster in this game, be it healer or dps-er. This happened in the detriment of the staffs that were dropping in the game and were d/e-ed or rolled on by alts only (or poorly geared players). If you are in a good guild, you would never take a Staff in pre-3.1 content.
Atm there are a lot of nice staffs, even more than in tier1 and I'm guessing Blizz will want them to actually be used by ppl, not d/e-ed anymore, so there are probably classes that will have BiS in the form of a staff this time rather than 1h+oh only.