Delta is offering a 25% bonus on miles transferred from American Express Membership Rewards through June 30. Registration required.

You should register for this promo, but not transfer any points. That way if you find you happen to need to top off a Delta Skymiles account, you don’t forget about this bonus and are poised to benefit from it.

But one should never give up the flexibility and value of alternative uses of Membership Rewards points for a modest transfer bonus to Delta on the if-come. Delta miles are too hard to use, and the program tends to extract too many points for redemptions, to make it a worthwhile place to stash your miles.

Note also that while points tend to post instantly when transferred from Amex to Delta, and though they promise two-day transfers, the bonus isn’t promised so quickly:

Bonus miles will be posted 4 to 6 weeks following the end of the promotion.

It may happen more quickly than that, but the terms and conditions suggest a sweep of accounts after the fact to post bonus points rather than instant posting.