The only downside to lowering tire pressure is the possibility of getting sand granules/dirt caught inbetween the tire bead & rim. This happens very easily when high "torque" (skinny pedal) input is applied as the tire bead will be slightly pulled away from the rim edge allowing sand/dirt to get trapped within the tire bead. This will cause a slow tedeous air leak when the tires are re-inflated as the tire bead can't properly seat against the rim due to the stuck sand granules/dirt. If this happens, the tires must be unmounted and both rim & tire thoroughly cleaned and then re-mounted-- a real P.I.T.A.!!!