Narrow band sensors are only 'accurate' between about 14.3: 1 and 15.0:1 and most cars with turbos run considerably richer than 14.3:1 on boost, so it would just indicate RICH most of the time, but not tell you HOW rich.

Wide band has a different voltage range and is accurate from about 7.0:1 to around 35.0:1 and will actually provide you with useful information.