GF
We don't have a procedures expert here.
I beg to differ on this point. You wouldn't believe just how many of us look into topics such as this and end up shaking our heads and moving to the next topic. I can't speak for TERPs procedures but, with PANS Ops, what do you think is the REAL significance of an IAF?
Here's a clue... IAS!
You have to start slowing down to the maximum speed for your category of aircraft. I agree that it isn't especially critical when you're under radar vectors but, from the point at which you join an initial approach segment - i.e. anywhere between the IAF and IF, you need to start slowing down a bit.
If you don't do that, you'll find yourself much too fast when starting the Final Approach segment. This is really important because WE go to great pains to ensure a reasonable ROD on final so that you can level off without busting a MDA, or without going thru a DA more than is provided for in the procedure.
That sort of thing can definitely ruin your day.