A rear disc is nice on a tandem since you'll have one rim you're not heating on a steep downhill (and we're talking 400 lbs going downhill), but the front brake is frequently a v-brake. In fair weather I can't imagine a single needing other than a good caliper with good brake pads; if your riding in the rain with a lot of grit on the bike maybe a rear disc would make sense. And once you've R&R'ed the rear wheel 2-3 times you don't even notice the disc; on the road you can turn a single upside down; we can't do that on the tandem and we still manage to fix rear flats.