The APG says "Armor Bonus: The number noted here is the eidolon's base total armor bonus. This bonus may be split between an armor bonus and a natural armor bonus, as decided by the summoner. This number is modified by the eidolon's base form and some options available through its evolution pool. An eidolon cannot wear armor of any kind, as the armor interferes with the summoner's connection to the eidolon."
Can anyone tell me why it would be better to have an armor bonus and not a natural armor bonus for an eidolon?