Logistically, fail-safes are expensive. Keeping a fire burning for days and years takes a lot of firewood, which in turn requires manpower. Having two fires als

Logistically, fail-safes are expensive. Keeping a fire burning for days and years takes a lot of firewood, which in turn requires manpower. Having two fires also places limitations on the distance they can be due to the angular resolution of the human eye. That'd be anywhere from 5-10 miles on basic assumptions. A signal fire can be seen from farther if you have the elevation and a smoke signal doesn't even require that (+ carries more information).