sbarner 

This lets you use the drum as it works best, to slow the bike over long descents and keep the rim brakes from overheating. Also, the cable pull for the rim and drum brakes is different, so getting maximum braking power is tough and prone to going out of adjustment. Finally, with mechanical systems, you are dividing the force, requiring a stronger pull on the brake lever to get the same stopping power at any single brake. This is the biggest reason most automobiles went to hydraulic systems in the 1920s. Some people will run the left lever to the drum and the right to the front and rear cantis, but that reduces their performance and doesn't allow you to direct more force to the front brake in a panic stop. Your setup is the best, as long as you remember to release the brake at the bottom of the descent! Tandems are one application where disc brakes totally rule.