I do always carry a back-up computer. Just one of my good old Palegic pucks on a wrist mount and attached to a D-ring. As mentioned, if my primary computer fails than my dive is over and I'm headed to the surface. Here in the PAC NW.....the vis can make it very difficult to control an ascent or perform a SS without access to basic depth data.. Even a basic analog depth gauge (which are basically free these days) is better than nothing...