Narrowband will clearly never be any substitute for wideband, and can not be considered as a tuning tool as such in that it won't indicate any sort of AFR accuracy except around 14.7:1 as already said..
BUT - if you take a wider view, it can be useful to tell you if anything is going wrong... if it's running rich on light load it will be obvious, if it's running lean on boost it will be obvious, by which i mean seriously lean... I've had a boost pipe come of the Fuel pressure reg. for example, or if the fuel pump has a big problem..