surely it is wrong to be dogmatic about this ,does it not depend upon the circumstances.
i recently saw two singers put on a good performance they were singing in harmony[which they did well] but they had words in front of them, the harmonies were not written out in front of them, but they were occasionally peeping at words, i agree it would have been better without words, but they were good, if they hadnt done a spot the evening would have been poorer.