The reason it has to be replaced is because it is bent. I don't know exactly what or how much, but I do know that I couldn't replace all the subframe bushings when I tried because the holes didn't line up. In addition, the right side middle body mount is twisted quite a bit. The guy at the shop said it could be tweaked back, but since the cost of a new one is so low relative to the work involved, may as well put on a new one.
I looked at the Wayne Due subframe -- no way I'm spending that much, nor am I going to throw away all the money and time I just spent on the front end already. (New shocks, springs, sway bar, brakes, POR-15 on the arms, etc).