I know how to change the oil in my car, so I do it. Some people don't, so they pay a professional to do it. An RMA is a professional service. Some people can do the application on their own and don't need one, others are overwhelmed by the process or don't feel like putting the effort in to understand it all (nothing wrong with that either) and so they pay a professional. Some people have complicated cases (medical issues, criminal history, children involved, etc) and can't navigate the process on their own.
By that logic, I can watch a youtube on how to change oil and figure it out. I would think there are some applicants who may not have a complicated case involving children, crime, etc., but just need a bit of a guideline and a booklet like this may be helpful. Not taking away from the services of an RMA when required.
Before I hire the services of any professional, I always research them, their history, and reviews from others who have used them. I don't need to know them personally to trust them with my money.
Not suggesting you need to know them personally, just stating that much of an RMA's service is by email (if one is offshore) and if the DIBP presents a requirement such as a Form 888, a booklet can give examples and recommendations on what to do versus what not to do in a similar manner than an RMA would suggest. Obviously an RMA will be reading your material and can make specific adjustments. But there are some applicants who get the 'jist' and may just need a slight tip, versus someone who has no clue what to write.
What's different between her guide and an RMA is you're not getting a service, only information that is already available for free. Her guide is not reviewing your application for mistakes, or stopping you from making one that can be fatal to your application. And seeing how it doesn't seem to be up to date and already claims false processing times in the sample, the information can be wrong. An RMA is up to date with immigration regulations and changes, future changes that may affect you, and has access to information that is not made free to the public.
Not suggesting a book will replace the services of an RMA. Apparently this book was somewhat generated by questions on a Reddit forum as per how I came across it. And it is time consuming to review hundreds of forum threads, so the money may be well spent, considering something as mundane as fingerprinting costs $60+ dollars. Since the DIBP changes the requirements frequently, a published document is not going to keep up to it. Incorrect information is doled out on this forum daily.
I don't see why someone does not want to spend "500+ quid" on a professional service but throws money at information that you already have free access to. But to each his own.
RMAs are not required, so don't spend money on one if you don't want to.
Not everyone wants or needs to spend the 500+ quid (not sure why you had to quote that - Brits refer to currency as quid, we're not all American). If they hire an RMA for full services, then they are in it for a lot more 'quid'. As mentioned, some people may have better things to do then spend countless hours on a forum, time is money.
Look the point of posting this here is I have quietly read these pages and those of other forums and see a lot of posts of people with simple questions, or from less wealthy areas of the world who may be struggling to scrape together enough money to pay the exorbitant fees let alone an RMA. Many questions posted here get replies from the RMA's such as "You might want to consult a professional to plan a pathway, etc." and that may be good and fine for some, but others may just need a couple of bloody questions answered. And as for the comment that the FB page disclaims any professional advice, that is no different than the signature on many RMA's replies. In fact any dealings with lawyers via email have some sort of disclaimer regarding professional advice.
There may be a third path, it's not just black and white as you seem to suggest - RMA or no RMA. It's just information, you may choose to be skeptical, but many people are suckered into using unscrupulous agents and may just need some simple assistance.