Mage lb has far more uses then dungeons just like someone already pointed out.

Also just as a note to some that may not know, Bard lb is actually stronger then caster. Obviously there are reasons you would use one class lb over another.
Before hw i really did not like that say smn, blm had the same exact lb same goes for drg, Nin, Monk
Now at least it "seems" different

Oh and yes PVP, don't you just get a 1 bar lb? I only recently got back into PVP so dunno.