My girls each have one (Felicity and Kit)---that was pretty much all they got for Christmas last year. They do play quite a lot with them--I've sewn a lot of clothes, and we got some hand-me-down beds and a horse, and they act out stories and have them watch movies with them, etc. We haven't bought any accessories. I also got a used Molly doll and cleaned her up, sewed new clothes, etc. for my little niece. I've never had any interest in dolls, but I sure learned a lot about doll restoration last year.
The other day they had a friend over and spent the entire afternoon making a doll movie, called the Adventures of Meena and Katerina. It was pretty hilarious.
Quill, if you look at the AG catalog, Gwen doesn't have any specific accessories to buy. Sonali doesn't either, though Chrissa has enough for all of them. I expect that for the short-term 'friend' dolls, they don't bother.