OK let me rephase it a bit.
A sim is not essential.
What is totally essential is to work out (somehow) on the ground exactly what one should be doing in the air.
This is true for VFR, too, though not as much.
In my IMCR training, I used to get silly stuff like a load of vectors and stuff; I had no idea where I was or what I was supposed to be doing, and only just before landing did I realise we were supposed to have been flying an NDB approach. I can do all the stuff now but spent the first 10-20hrs being totally baffled. You cannot waste the student's money like that. One needs a thorough ground briefing, with "to do" points written down, and understood, and only then should one go flying.