The pole doesn't really need to be plumb for fixed-aim.
But for a motor, it does need to be perfectly plumb to track the arc, as PhlatWound mentioned.
Also, the extra off-balance weight of the motor, -and- the fact the dish is now further from the pole center line, both conspire to drag the pole off-plumb.
So, a short, large diameter, stiff pole, in concrete, would be the best security for anyone putting in a motor.