I think the original intent for the inclusion of multiple races was simply for enemy variety and little more. It's more fun to shoot at various enemies with different abilities and weaknesses than the same set of guys every single firefight. As the game's popularity exploded and Bungie got a chance to elaborate on Covenant society I drew the conclusion they were more like the Roman Empire (in SPACE!). They expanded through conquest and assimilation, using the culmination of their religion as a uniting goal.
The games give us a very limited view of the UNSC, because we only ever see it from the view of a soldier. The supplementary materials (the novels) give us a lot more to work with and in a way, yes, the UNSC does de-emphasize the importance of the individual in favor of a more streamlined, uniform society. This much I believe was intentional on Bungie's part because it tells us that the UNSC are not a universal force of "good" and that there are quite a few things wrong with this kind of society (it just happens to work when there's a war with an alien entity, which is all the games are able to show us). While the SPARTANs themselves are ethnically and culturally diverse (like the colonies) from the outside you can't really tell the difference between them. The SPARTANs seem to be the perfect analogy for what UNSC is, and also what it needs in this time of crisis.
The games give us a very limited view of the UNSC, because we only ever see it from the view of a soldier. The supplementary materials (the novels) give us a lot more to work with and in a way, yes, the UNSC does de-emphasize the importance of the individual in favor of a more streamlined, uniform society. This much I believe was intentional on Bungie's part because it tells us that the UNSC are not a universal force of "good" and that there are quite a few things wrong with this kind of society (it just happens to work when there's a war with an alien entity, which is all the games are able to show us). While the SPARTANs themselves are ethnically and culturally diverse (like the colonies) from the outside you can't really tell the difference between them. The SPARTANs seem to be the perfect analogy for what UNSC is, and also what it needs in this time of crisis.