If we continue to pretend that women aren't in combat, and close some roles to them, we deny them the promotions that go to the men they fought next to. We perpetuate the myth that women aren't really warriors, which, in the military, means you are worth less.
Seemingly out of nowhere, the Pentagon announced that it would lift its ban on women in combat. Just like that. No protracted national culture-war debate, no threats by conservatives to block progress -- in other words, nothing like the fight the nation saw over letting gays serve openly.