A narrowband O2 sensor is only good for a very narrow range right around stoich which is not really of any use under any kind of load. Any bossted engine under load better not be seeing an A/F anywhere near stoich or it wil be history. As such, knowing that a boosted motor tuned at say 12.5:1 under load is running at perhaps 13.5:1 could save the motor whereas on a narrowband set-up it would still just be displaying rich while things are on there way to melting. Also, a lot of cars use wideband O2’s as their sensors and as such the sensors are generally pretty cheap and will have similar lifetimes to narrowband o2 sensors, unless of course leaded fuel is being used.