But this rich pool of xs' is needed to have certain amount of builds possible. And probably if someone can bind more variables to his build it becames less linear than the other builds and thus will probably become unbalanced...
So the question is isn't rich variety of builds possible higher value for the player than the perfect balance?
variety is the spice of life.
true "perfect balance" can only be achieved when all competitors are identical - like in chess, each side gets the same type and number of units - just one "build" possible for your "army" when you play chess.
one should not reduce the number of types of builds possible to ensure balance. only when the build is way overboard, like having a nuke option in Oblivion, "Someone drop us the bomb!" <g>. a class that can use tactical and strategic nuclear weapons in a fantasy rpg would be even more unbalanced than having guns.
But its possible to twist things so even guns and bombs and magic can be relatively balanced. i once expanded the AD&D rules from +5 to +10 magic, then expanded the Traveller rules from tech level 13 to tech level 24, then combined the two into one huge rpg game. the D&D world was just one planet in the Traveller universe where magic happened to work. A pc version of that game would be wild! the custom rules on top of the traveller and ad&d rules was a stack of papers a foot and a half tall! once the conversion rates between the two gaming systems (how many traveller hit points = how many d&d hit points, etc) were determined, it was all easy.
so go for the maximum possible number of "parts" to create "builds" from. this gets you maximum variety.
then just do a sanity check so that no builds are too strong or weak. buff up the weak ones, tone down the strong ones. but keep the naturally weak somewhat weak, and the naturally strong somewhat strong.
perfect balance is not the goal. the goal is to avoid excessive imbalance between a wide variety of builds which are inherently of different strengths to begin with.
different builds will be inherently better most of the time, when there are many to choose from.
a strong combat build will be good most of the time.
a "merchant" build will naturally be weak, but can get rich more easily.
as a result, things are like in reality. a merchant would never confront a fighter, he'd hire an assassin! <g>. a perfect example of the dynamics of three different builds there (merchant, fighter, assassin).