The main disadvantages to using BRD as a sub is the lack of ability to equip an intstrument, and you may only stick one song effect at a time. Some of the things you mentioned do have merits, but I can't expect any debuff spells to have significant effect due to the halved skill levels.

You mentioned SMN, which is a pretty good choice. Unlike /WHM for /BLM, the only real advantage BLM adds is the boosted MP pool. You could argue for the Conserve MP passive trait as well, but that's kind of a crapshoot. SMN is good not only because the even higher MP boost, but also at level 50 you'd get auto-refresh, which is pretty handy.