Allies in MA missions will reduce your XP earned if they damage the same mobs you do. This is different from allies in regular missions. MA allies are like Confused/Deceived enemies or players on another team, while allies in regular missions are like pets.
This is to prevent the obvious exploit of letting allies do all your fighting for you to gain xp risk-free. However, if the ally is keeping aggro off you it may be worth your while to have the ally, for reasons akin to Illusionists and Mind Controllers/Dominators using Deceive and Confusion.
Allies that do not attack but only buff the player will not reduce your xp, but could be considered exploits. Many farming missions include multiple buff-only allies (FF defenders and kins are favorites), and are obvious exploits. You should tread carefully.
Does including allies in your mission reduce the rewards (xp or tickets)? Does it make a difference if the allies do no damage (buffers for example)?