Doesnt really work with 40k though.
say you play chaos, but want a terminator themed list?
well the cost difference in terminators compared to cultists means that you are unable to have a significant portion of your force be terminators in a standard 1850 game.
Percentages put huge constraints of themed lists that do not have core troops that fit the theme
EDIT:
one marine specific example.
under likely percentage rules (from the recent BoLS article) a space marine terminator squad is not allowed to purchase a dedicated land raider in games of less than 1850 (and not even that with certain load outs)
stupid idea
edit2: All percentages are llikely to do is drive the game even further to hyper-efficient builds, in poarticular ones that rely either on a single hammer unit that draws its percentage allotment from multiple areas (HQ, ELITE, ALLIES) OR towards spamming effective troops choices and not considering anything else beyond a few force multipliers
edit3: so essentially percentages in 40k would
limit possible themed builds
hinter entire units structures thus driving the size of the game to higher points to include them
not likely combat the 2 perceived problems of spam or deathstars