Well how about just when the're in your sights?
Players move so rapidly, and there can be so many on your screen at one time, that being able to instantly distinguish friend from foe when players are swarming around a flag is the only way to gain the situational awareness you need to be able to make your targeting selections. If you had to hover over every single player to see whether they're a friend or an enemy, you'd never be able to make decisions quickly enough to be effective.
Now, having friendlies marked, but enemies unmarked, is an option that would still allow you to tell friend from foe rather easily, though it would make identifying enemies a LOT harder. But if we're marking friends, it only makes, well, SENSE to mark enemies as well (even if they aren't marked until they're much closer).