The need for each spouse to be Platinum Elite is that, based on the MR Ts & Cs, your wife is not actually eligible to receive all the benefits of your status if you are not staying with her.
As you say, she does receive these benefits in practice, but really she shouldn't.
(MR does not offer a "couples" membership, which I believe it should, but that is another matter)
http://www.marriott.com/rewards/terms/elite.mi
Benefits of Elite Membership are reserved for Members only and apply only to the one guest room in which the Elite Member is staying. As proof of eligibility, a Rewards Program Membership Number is required when making reservations.