They would not have to balance, if you went with the fluff, you would just have less of a unit, or make them more expensive. And to make the armies different chaos has lost a lot of good things they should have to the space marines. In this game their is no superior race, they are all designed to be balenced, each are strong at different points in the game. It's all about how good the player is with their race.
For me the cycle goes like this: SM>Eldar>Chaos=Orks>IG But it really depends on the race you play.
orks are kinda weak but at the same time not they cant really kill much all you can do is HAMMER THEM TO DUST but come to think of it most of my fictories dont come from me killing them it comes from my allies while i just throw shit at them i ALways have at least 200 deaths
Chaos sucks except against orks, im best with sm,then eldar, the only reason ig is good for is their arty i love earthshaker ability! I have to say da hardest race to play is orks. If your defensive then offensive try sm or ig if your just offensive try eldar
I think it has more to do with the amount of skill and micro required to use a race successfully than balance per se, though it can be argued that this is an issue.
Hands down, SMs are the easiest to use. Space Maries are a great all around sqaud that are easy to specialize for needed roles, and thus easy to customize against various races. Add in the possibilities with grenades + rockets against infantry (bouncing FTW), as well as their strong vehicle support, and the only thing stopping the SMs are their overall higher costs.
I've played Eldar the most in a general multiplayer setting; they take alot of micro, and it seems to me to be a bit harder to climb up their tech tree than with other races (soul shrines are expensive). They also seem to require ALOT more research upgrades than other races (or at least Marines) to bring them upto 'snuff, meaning that it's harder to balance your army vs. needed upgrades (like Mystism + nades for Gaurdians, Fleet of Foot, various Webway Gate upgrades, NM'ing upgrades for simple unit production, or Psi Storm to really get the most out of your Farseer). They also require more thought in infantry compisition. But I'm new to the game, so take this with a grain of salt.
Chaos is the only other race I really use on anything resembling a regular basis. I've got their early game down decently, generally mixing in 2 - 3 cultist squads decked out with grenade launchers (again, bouncing FTW) supporting Chaos Space Marines, though I haven't found the "perfect" infantry combo against various races.
I really think that in the beginning, Orks are really weak. The Squiggoth, however, is probably the best unit in the game, making up for it, as long as the Orks can last long enough to build one... Also, SM do have an unfair advantage due to the Orbital Relay and the Orbital Bombardment (mainly, i think, because buildings cannot, generally, move), and Chaos used to be pretty good as well (until WA removed everything except the Heavy Bolter from the Marine squads, and removed all the upgrades from the Chaos Predator). Also, I think that the Chaos army has become too close-combat oriented, meaning it has no real answer to a strong, very mobile force (for example, Eldar, or SM). The Eldar almost seem a bit too strong, with their aspect warriors and vehicles, apart from their lack of artillery. The Avatar is a great unit, you can keep pumping out units as long as its alive, meaning you have an almost never-ending supply of Dark Reapers against SM or Chaos, and warp spiders against Orks...