I understand kids getting them but I do think it's odd when adults get them and there's no kids in sight. I'm sure there is always a reason, though.
The only times autographs frustrate me is when I have found a relatively short line at Epcot for my 2 year old to get in to meet Minnie, and by the time all the kids with autograph books get them signed (many who don't have their books open and characters fumble for a blank page) and then have pictures taken, Minnie has to leave before my DD meets her. Not easy to explain to a 2 year old. I know the kids love their autographs, but there are times when I would love to see the CM see that only four people are left in line but the character has to go in a minute or two and would say: "To accomodate everyone in line, we can only do pictures now. Sorry."
I did bring a blank book for DD on our last trip and she had the princesses sign the book when we were at the PSB lunch at Akershus and when we met Ariel in her Grotto. She looks through the book now and loves seeing the autographs. I plan to put them in a scrapbook I'm making for DD with pictures from our last two trips.