Observations from the newbie. In contrasting what is often taught vs PMTS's focused approach, I noted in an earlier post that the major difference between much teaching and PMTS is how the ski's are turned. In many TTS's, its outside leg steering, while in PMTS it's inside leg tipping. This stance issue came up in the recent tips thread, but it comes up all the time on other forums and seems to be another, perhaps, hallmark difference between PMTS and many traditional ski instruction methods.
For me, in my experience, wide stance does not equal stability. If my skies are wide I am stable just standing there, but once I start skiing a wide stance makes subtle shifts in balance impossible. Balance shifts must be made with whole body movements rather as responses to inside leg tipping movements. With a narrow stance and one ski balance, the CM moves from the inside of one turn to the inside of the new turn by moving a matter of inches. With a wide stance, the CM is not only harder to get to move, but must move farther. (try balancing on one leg standing still with both a narrow and a wide stance, the wider the harder)
The other experience that may seem counter intuitive as far as stance and stability goes, and this has been a big one for me, with a wide stance in uneven terrain, bumps and crud, you are more likely to be fighting with each ski trying to go it's own direction as each ski is on terrain that is different than the terrain the other ski is encountering a mere 1 foot away. This is common sense. The closer your skis are the more likely to be on terrain that is reacting with the skis in a similar fashion. So, bumps and crud are easier to ski with a narrow stance and with one ski weighted than with a wide stance. In a wide stance with weight balanced on both skis you are much more likely to lose balance as one ski goes its way and another ski goes its different way.
I have not skied much powder (just one day so far), but from what I have read in both Harolds, Lito's, Craig's and Eric and Rob's book all recommend a narrow stance with more evenly weighted skis - making the skis work like a monoski - sort of.
Personally, I have had nothing but trouble from anything in my skiing related to a wide stance. I wonder if the current trend to ski with a wider stance comes from misintrepeting pictures of racers. In most pictures of high end racers, their legs are together, but their feet are apart. This is a narrow stance but just inclined.