Originally Posted by thepointsguy

Delta branded Amex' don't earn Membership Rewards points- they earn Skymiles. Membership Rewards points can be transferred to 16 airlines, most instantly. This promotion targets those customers.

For what you give up in transfer flexibility for your Reserve card, you have the ability to earn MQMs- something that the Membership Rewards cards do not offer.

As for your Amex Blue- those points are pretty worthless- I'd upgrade to a Gold Card today- they may even give you a deal and waive the fee for the first year, like they are doing with this offer.