We don't disagree, I think, Foulestroupe. It's not really different from the position in regard to reading print. If you can't read print you are at a serious disadvantage. But if you feel you can't sing without reading from a book, you are also pretty disadvantaged.

I distinguish between needing to read the words or notes on the one hand, and on the other maybe feeling more comfortably with them there, just in case the memory slips.