Show me the studies. No one has done a single study on DADT and its effects.
But I will tell you one harmful effect. The military community knows that they are being used as a political wedge to force gay marriage on the rest of the country. Whether they agree w/gay marriage or not, the fact that the military is being used in this and other cases as a social tool is demoralizing. Many of our best young men and women are quietly getting out of the military, leaving behind those who are perhaps less capable. And it's not bigotry that's doing it. It is, rather, a rejection of that political role.
I know this because I'm a spouse, and I've seen what is going on. Most of America will not know this for years. Watch and see.