The other problem with using the same spoke length everywhere in the rear is that it puts an unnecessary wrinkle in the process of dishing the wheel. You're going to want to thread all the spokes in the same amount and then keep turning all the nipples by the same amount until you're ready to start actually truing and dishing. If you do this with spokes that are close to the correct length, the uneven lengths gets the dish headed in the right direction. If you use spokes that are slightly too long on one side and slightly too short on the other, you'll have to make extra turns on the long side to get the dish closer to correct. That's not a huge deal, since you have to get the dish right anyway, but you can either have the spoke length working for you or working against you. Given the choice, why not make things easier?