Well, If you don't use them & just mount your springs solid to the frame your rear springs will bind up. So NOT using them would be a disadvantage. But that's probably not what you meant.
If you're asking about using longer ones to raise the rear of the car, yes there are disadvantages. They raise the car's center of gravity, they put undue stress on the springs, they have an adverse effect on the handling of the car.....etc. My advice, don't do it. But maybe that's just me. :tongue1: