Definitely great discussion...
There isn't a right or wrong answer here, it's to each their own.
I certainly get what you're saying about the holistic player valuation. But for me I want to look at a the sum of all parts to generate a better holistic player valuation. I definitely agree with you on the argument of "value to my team" as opposed to "player value", I think there are many people out there that confuse the two. Much like "upside" and "actual production"...
I think z-scores are great since it's relative, but I think z-scores might need to be looked at from each individual stat perspective so a z-score for each of G, A, +/-, PIM, SOG and PPP rather than an amalgamation of all of the stats put together to generate an overall "player value". If you look at the Kessel example, he's pretty good at 4 stats, but because he's very below average in 2 categories, he's a 76th ranked player. I mean do you drop him 50 spots because of 2 weaknesses?
But I could see 2 different scenarios as well, H2H Kessel is much more valuable because he gives you an advantage in 4 of 6 categories, which is the key to winning, if you consistently win 4 of 6, you'll be lifting your league championship... but I wouldn't want him in Roto because his lows are so "low" that it's counter productive to own a player like him in Roto. You can't win a Roto league with top scores in 4 categories, but 1's in the other 2...
So league settings play a role in player values as well.
Dobber Expert Pool Champion 2011-12