In terms of one or two chapters, I guess it depends on how many chapters are in the book, how long those chapters are, and how central those chapters are. If it's a ten- or twelve-chapter book that the instructor would not normally require the students to purchase, especially if the library has a copy or two in its collection, I think it's very reasonable to put those chapters on reserve.
It's very difficult in some courses to find the exact, precise textbook you want to use. As you go up the educational hierarchy, from first grade to twelfth grade to freshman, and all the way up to graduate school, it's harder and harder to find that one text book that fits, so instructors are often in a situation where they have to weave things together.
Fortunately, more and more things are available digitally, and this is where those huge library expenditures come in. The libraries are trying, as much as possible, to figure out what people need so as to minimize the need to even rely on fair dealing. So much more of the readings for a course could be pursuant to a licence, a broad site licence, that everybody in the university has access to.
I think the way out of this situation in the long run is to give our libraries better funding to empower them to do good collection development work, figure out what teachers need to give to their students, and purchase it. Fair dealing is there, though, for those situations around the edge, and sometimes you need to copy a chapter—one good chapter from a book—that you wouldn't otherwise ask the students to purchase.