The NB calculator comes pretty close and for most beers it's close enough to calculate ABV and to know whether or not you yeast is working and if you are ready to bottle.
The calculators issue is when you are brewing beers with high OG - as the adjustment in the formula is less accurate.
If you are looking for lab precision, then yeah use a hydrometer. If you want to know if you are ready to bottle, the refractometer will do.
It's been my experience that the refractometer readings (using the NB calculator) have been in line with my software's predictions and calculations consistently enough to not replace my broken hydrometers.