I agree - it's important to use the textbooks.
The ActEd Notes aren't designed to avoid the need to look at the textbooks - they're there to complement them, to guide you through them in a sensible order, to highlight key areas to focus on and to expand on areas where we think the exam might require additional knowledge.
We've taken the view that there's no point in simply repeating the same detail as in the textbooks where they do an excellent job all by themselves. So by not looking at the textbooks, you'll be missing out.
However, the textbooks weren't written with actuarial students preparing for the ST6 exam in mind, so the focus isn't always totally aligned with what's required. In such cases, we deal with this in our Notes. So, it's probably not a good idea to just read the textbooks either. Both the testbooks and our Notes are needed to ensure a full understanding.
Having said that, I should probably admit that I'm aware of people that have passed ST6 using just the textbooks and others that have passed using just our Notes. I regard these strategies as being high risk though!