MIF has questions that are easier than year 10 textbooks. Sadly a lot of schools use it because the students 'aren't good enough' to use harder textbooks and as a result, students get bad exam results. This continues pretty much until the student gives up and drops to general, or starts using a more challenging textbook.
When schools assess that students aren't good enough to use Cambridge, they are usually correct for the majority of students.
The fact is that large numbers of 2U students don't have the ability to understand this maths beyond the most basic level, and MIF is all they are capable of handling.
Those students would get even worse exam results using Cambridge alone, because they will never cope with the harder questions, and don't get enough practice at the basics.
Of course this creates issues for the handful of stronger students in the class, so the teacher should be supplementing the homework with questions from those books.
Yeah exactly
Think about it, if people aren't challenged, will the world even progress?
More often than not, people (and the world) progress when they challenge THEMSELVES.
The problem with that 'handful of stronger students' ... the majority haven't worked that out yet. They are prepared to go along with whatever is presented to them.
(Yes, I know I'm generalising .... there is no need for anyone to say "but I am not like that")