I think simulator is an important ingredient of IR training. A lot of time and money can be saved by practising the procedures in the sim. If you make a mistake, you can be repositioned immediately. The instructor can set up situations and you can practice drills. At my FTO, they allowed me to practise on my own in the sim free of charge.
You are right that there is no substitute for the plane especially in terms of ATC, weather, unexpected gusts of wind and traffic but I think it is good to try everything out on the ground before you fly the plane.
I did all my IR training on a twin. You need to get thouroughly familiar with the complexities and speed of a twin so personally, I wouldn't have though there is that much benefit in doing training on a single.
Hope this helps!