BU let me in pretty quickly and I accidentally didn't write a PS on what life events had made me want to become a lawyer (the specific topic they asked for) and I am pretty sure that they actually read the PS because there was a handwritten note about it at the bottom of the acceptance letter.
I wouldn't even apply if I were you. It's late in the season, so your chances would be lower as it is. A subpar PS would just stack the odds further against you. Plus, you said yourself you probably won't go there. Based on some other threads, you seem pretty psyched about USD anyway. Save your application fee and put it towards the insane cost of living in SD .