I think reading Roadcraft will no doubt benefit anyone wishing to undertake any advanced driver training. It may also help clarify points where other manuals such as PYADT or How to be an Advanced Driver are not totally clear.
For IAM your not being tested on it.
For RoSPA and HPC you are being tested on it.
If you ask the IAM, your being tested on the IAM's manual not Roadcraft (most of it comes from Roadcraft anyway). In fact any IAM observer should tell you that.
I think Roadcraft goes into more detail in some areas that HTBAAD (the new PYADT).