The cases in your book are edited to focus specifically on the subject of the day, if you will. The advantage to the edited version, it's shorter, and you have a lot to read. The disadvantage is that because it is edited you can miss out on the structure. Actual elements might be ommitted. The procedural posture will be confusing at best.
I was actually just thinking about this today at work. I often use the headnotes to get right to the part of the case I want. But I read the the lead cases all the way through. And I think it would be less confusing in the end. I understand the things like the standard for summary judgement so much better now reading them in context and applying them to the facts. In fact, I really didn't "get" the point of lawyering until I was reading whole cases for legal research and writing. In the end, you need to know how to read the cases, because that is what you will do as a lawyer. But you also need to know when to skim or buy the High Court Case Briefs (ConLaw- even reading the casebook all the way through is likely to kill you. But I did still the unedited case for lead decisions).
Other issue, there are small case summaries and notes in the text book. If your prof uses theses, and questions from these, you need to at least have a friend from whom you can photo copy.
But honestly, you can always buy a copy later if it is clear you need them. It is a ton of money. Most of which you will never get back. So if you want to, I say go for it. Especially if you will have westlaw access from the get go.