The Ottoman, Greek and Roman Empires all had gays serving in their armies, and they managed to conquer much of the known world.  Keeping gays from freely expressing their sexuality in today's arm puts them at risk for blackmail and bias crimes. 

Critics of the "don't ask / don't tell" policy point to the tragic case of Barry Winchell in 1999.  The gay army soldier was beaten to death by a member of his platoon.  The Servicemembers Legal Defense Network reported 400 cases of anti-gay harassment in the military in 1998, including death threats, compared with 182 violations in 1997 - a 120 percent increase.