2015-08-25 : 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 they don’t, Reckless Cohort doesn’t have to attack.

2015-08-25 : Reckless Cohort can attack if you control another Ally. It just isn’t forced to.

2015-08-25 : 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.