For myself I buy whatever I need whether it’s a piece in a collection or a single piece. For students it’s another thing entirely. Some of my students have parents who will buy a book as soon as I tell them to get it. A lot of them though are rather hard pressed and expect that whatever book they get should be enough for a long time. So for most of my students I like individual pieces because then I can give the parents a list of things to purchase once or twice a year and the pieces don’t cost as much as the book so I can probably write down one book of pieces plus a few separate solos whereas I know the parents will only show up with some of the things I’ve asked having been purchased if I write down several books because of cost. Therefore it’s helpful if I can get solos separately because otherwise these students won’t be playing those solos.