"Balance" is such a subjective term, and it's because of the varying definitions of it that people have arguments like this one. To some (ie Bungie and the OP of this thread) balance comes in the form of a weapon being decent enough to garner kills but still weak enough that it rewards players who seek out and obtain better weaponry. To others (ie MLG and the antagonists in this thread) balance comes when a spawn weapon can effectively combat all scenarios.
They are 2 very different definitions but depending upon which you prescribe to, your "game" might be balanced or it might not. Because the people who prescribe to the former's definition of balance see the latter's as unbalanced because the BR is too strong of a spawning weapon. You can choose to use other weapons, but you'd be better served by strictly staying with your BR. It's the 2nd most effective weapon in Halo 3, and it shows that. Which is why a game of Team BRs has definitely less "weapon variety" than a Team Slayer game. To the people who subscribe to the first definition, a weapon start that decreases variety and interferes with the flow of the weapon sandbox is unbalanced.
Now, if you subscribe to the second definition of "balance", than an AR/M6G start is unbalanced because it leaves you vulnerable to opponents who have superior weaponry off spawn. Having a BR spawn decreases the benefit of seeking out better weapons (which is why most proponents of BR starts like them) and seeks to cater the game around a weapon they consider "skillful" (ie MLG's argument for it).
For the people who talk about spawn camping AR-starts on Last Resort like that scenario has anything to do with weapon start? It has far far more to do with spawn issues than weapon start. I've spawn trapped people with AR-starts on that map and I've spawn trapped people with BR-starts on that map. To me, it's easier to spawn trap with BR starts because even if you manage to kill one of the people spawn trapping you, they'll be back over to their previous location with a fresh BR in 20 seconds. In an AR-start game, they'd have to spend the time looking for another BR which could give you more time to break the spawn trap of the remaining players. Spawn trapping on Last Resort has never been about weapon start. It has always primarily been concerned with bad spawn issues.
Now, if you care enough about my opinion, what I see "balance" in Halo 3 is AR/M6G spawns on any map(s) that doesn't have an offensive vehicle. Anything that doesn't fit that criteria can be BR/AR spawns. Because the strength of the vehicle can help counter the increased strength of the weapon spawn.
Now in response to how AR-starts do or do not "persuade" you to seek better weapons, BR-starts "persuade" me to use nothing else because it's more effective than 96% of the other weapons. I strictly use the BR in BR-start games because I am forced to because my opponents are spawning with the same highly effective weapon. I'd put myself at a severe disadvantage to use something else less effective. I'm not disagreeing with the sentiment that in AR-starts you aren't essentially "forced" to find something better than the AR; I just don't see it any different than in BR-starts you are essentially "forced" to only use it because of it's effectiveness.
~B.B.