I would think that in those cases the partner-card has zero value for those that travel together. And for Platinum-for-life people, the the main card comes only with the extra miles as a benefit. I wonder how many people have "sold" the extra AF/KL Platinum level.
To add insult to injury, KLM in The Netherlands has just significantly increased the fee they charge for their own co-branded card. What remains is an expensive INSURANCE card, because in the AMEX world the card is not recognised as AMEX Platinum; it's more like AMEX Blue with insurance..
I have been told (and thus i may be wrong) that the large difference in fee between gold and platina, is caused by the comprehensive car rental insurance. However, in the new FlyingBlue Reinvented scheme, the differences in Experience points earned with gold/platinum are minimal.

Having said that, I do find the service from AMEX very good and also claiming mishaps from insurance is easy and claims are processed quickly.

How much i would love to see a good review of travel insurance available for frequent flyers.