Reckless Cohort can attack if you control another Ally. It just isn't forced to.
Reckless Cohort checks whether you control another Ally as attacking creatures are declared. At that time, if you don't, Reckless Cohort attacks if able.
If, during its controller's declare attackers step, that player doesn't control another Ally but Reckless Cohort is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under that player's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having Reckless Cohort attack, its controller isn't forced to pay that cost. If he or she doesn't, Reckless Cohort doesn't have to attack.