On a non-ghost type is there ever any reason to use curse when it can learn bulk up? The only reason I can think of is if your lowering your speed for gyro ball or to move last for double power with payback
Yes, the only real reason to use Curse over Bulk Up is to boost the power and effectiveness of Moves like Gyro Ball and Payback.
Trick Room would be a reason to use Curse instead. The speed reduction from Curse would effectively be a speed increase during Trick Room.