IMO it's really pretty simple, and I can't believe there are this many posts about it! So here's the short course:

If you like to sing with a songbook, go ahead. If you don't, then don't. Why go on from that simplicity, and judge others' choices about the matter?

Besides, as we all age, some of us are going to confront memory problems and may need songbooks. Some of you who are judging people who use them, now, may be judging people who have simply reached that point before YOU have. IT'S NOT YOUR BUSINESS.

~S~