SE must needs (lol) make ranking apply on a per-job basis just as deep dungeon and Ishgard restoration scores were kept.

For the purpose of Top 300 rewards, only your highest ranked job will be considered (you cant recieve multiple T300 rewards for achieving the appropriate rank on multiple jobs).

This will also allow the players to see who is the best at "x" job and encourage more ques as people also compete to be the highest rank on thier favorite jobs. They could even introduce new titles and achievements based around specific job rankings. (Deadliest Lance = rank 1 DRG, Tyrannical Lance = T30 DRG, Stardiver = T100 DRG, for some examples)