HWD,
i agree in principle with what you have stated. My only arguments for the use of a simulator as opposed to using a simple or complex single for the early element of the course is when introducing a new concept to a student as is the case with the IR (in particular when the student has only the minimum experience on instruments) are:
1. The ability to "rewind" and start again at any stage, if you screw up an approach in the air, there is either the climb/return to the beacon or re vectoring to the start of the approach. If you're very unlucky this can start to get expensive, especially if you end up getting re-sequenced behind a lot of other traffic.
2. As you mentioned, in the real world there is real weather and real traffic/communication chatter. In the early stages of the course when all the ideas of the exercises are not ingrained enough to release some spare capacity these will be an unwelcome distraction. As you get further through the course you will develop the capacity to not just be focused on trying to do the one task in hand.
Ultimately there is no substitute for real hands on experience in the real world, after all, you will not be doing your initial IR at least, in the simulator. but for the builiding blocks section at the beginning of the course i believe the simulator is a useful tool.