IODP 1.1 will use simunition this year (I think they have for a couple of years now). (NB: IODP 1.1 is Phase III, dismounted platoon commander's course).
FWIW, my first experience with "simunition" was as a pasty civvy playing paintball. Now, I KNOW that this isn't the same thing, but there was at least ONE lesson that MAY translate.
For the very first game (battle inoculation?), we were in an open field with some barrels (the big oil drums). We lined up and had virtually zero cover. The aim of that game wasn't to win, but to get shot, to get over that first fear. Unfortunately, I avoided getting hit. I was hit on the very next game, in the throat, from about 10 metres. Frig, did it ever hurt! Anyway, why did I get hit? I peered OVER cover (instead of beside it or through it) and Zippy from the blue team nailed me. What was my lesson? Don't crest yourself. I think that was the last time I did that (at least while playing paintball!)
So, in reading through this thread I think that there are a few things to consider
The 5.56 is perhaps a "general purpose" round, as it is designed for use at all ranges. As mentioned, until we have smart bullets that can alter their physical state at the muzzle, we can't go mixing and matching bullets. But, something must "alter". What would that be? Well, for shorter ranges, where a fatal shot is NEEDED, we need to train to instinctively shoot "the magic spot". For longer ranges, shoot as is currently taught? I mean, if the bad guy is at say 250 metres, a near fatal shot may be enough to incapacitate him until you close with and destroy. Heck, even if it just suppresses him, you've done the trick.
So I guess it comes down to realistic and relevant training, including simunition augmenting live fire.
Too bad we can't shoot our own with 5.56 mm in triaining to get rid of the initial "fear" of bullets (JUST KIDDING)