IMO if you fly IFR and airways you have to be able to fly instruments with the precision and tolerances required in the JAA and FAA instrument ratings! Do that and you are fit to fly IFR /airways whether you are a PPL or CPL but there has to be a challenging flight test which will load you to make sure you can handle the worst / I never had the impression that the IMCR quite made that standard ?
Actually do you need to fly to that precision and tolerance. I know we are taught to think we do but in reality I wonder if some of the tolerances are unnecessary.
I also doubt the JAA flight test in particular tests your ability to cope under real world pressure - it might teach you to cope under exam pressure, a rather different think. The FAA flight test possibly does a much better job.
The evidence is IMC rated pilots do a pretty good job of coping with the conditions they need. The easy bit is the en route bit so combining the two is unlikely to alter the stats, in fact it might make them better because it just plain stupid to be forced to sit in the weather which is so often the consequence of the way the IMCr is currently framed.