The Home Instructors Guides are necessary, in my opinion, because it teaches the concepts in the "Singapore way" of learning math. This is what makes it a unique math program. If I would have just bought the text and workbooks and said let's see how we do, we might have been just fine, but again, the CONCEPTS are necessary to be learned early in order to help the student think mathmatically and truly understand how math works.
You'll notice, that if you look for the Home Instructors Guides in the Sonlight or Rainbow Resource Catalogs they don't have them for 1A or 1B. I had to purchase them directly from Singaporemath.com.
Also, be sure you get the Home Instructor Guide, not the Teachers Guide. Hope this helps!