It all depends on how you define entertain. Something along the lines of holding onto your attention, maybe? For me you don't have to have showmanship to entertain - I will be very much entertained if I watched a good musician (with less colourful personality than others), maybe the said Norah Jones, play, because of her good music.
Some people probably have another defination of the musicians' form of entertainment, i.e. in bands where members would actually prefer vocalists with more showmanship, rather than the stand-on-one-spot singers.