In rope access we use a second system for two main reasons; to protect against main rope failure, and (more common in practice) against user error of device on main line- eg death grab on a stop descender. These shouldn't be such a concern climbing as a traxion is fairly foolproof, and you can position the rope as you ab in (take a rope protector or two) and it isn't constantly in tension.
When I tested the Ushba rocker-style device back in 2001 it was one of only two on the market which had the potential to completely chop the rope on a fall. They may have redesigned the cam since then.
Sometimes when photographing I'll use a double rope system, mainly as the backup line can be rigged off to the side to help positioning, but also for peace of mind (aching in to Longhope was terrifying!). I use a Shunt or more recently a Kong backup, which is quite a neat little device.
Most climbers are nervous about toothed devices damaging the sheath in a fall. This is actually very unlikely as the fall factor is so small, but it's a good reason to use a low-stretch rope instead of dynamic as they have a stronger sheath and will stop the fall without a load of rope stretch.
