If your primary usage of the Curve card was to be able to use your American Express at more retailers you probably want to understand the economics of each of the wallets.
Unfortunately my understanding is that the economics of a Blue or Black Curve card are pretty bad.
Bear in mind that these calculations are slightly different depending on the earning rate of your card (Premium Plus would be an exception since it earns 1.5 avios rather than 1 avios/point).
Anyway, here goes…
If you have a Blue Curve Card or Grandfathered Black Card?
Since the cost of loading money into the card is 0.65%, that means that if you load £100 into the card it will cost your 65p.
American Express cards generally offer 1 point per £1 of spend (with all of these calculations, I’m assuming an American Express Platinum — the actual earning rate depends on the card). At best you will probably get roughly 1p out of every 1 point (see: https://www.headforpoints.com/2018/12/03/what-is-the-best-use-of-american-express-membership-rewards-points-2/).
Therefore, normally spending £100 will get you 100 points, and this is roughly £1. However, in this case we need to subtract 65p from this since you must pay the cost of “loading” into the Curve card, and once this has been done it turns out that £100 of American Express spend produces only 35p of reward point value.
To illustrate why this is bad with a comparison, there are a number of non-Amex cashback credit cards on the market which offer 0.5% cashback (e.g. https://tandem.co.uk/credit-card). These make you 50p for every £100 you spend, while an Amex behind a Curve Blue card will make you only 35p (and this is also not fungible or earning interest).
What about a Curve Black card?
This will cost you £120 a year and provide a number of other benefits which you may or may not want. If you spend the maximum amount of free Amex a month, you will get 12,000 points. If you spend these points carefully you will get £120 for them. Since that’s the cost of the Curve card, you will just about pay it off, and any spend over this per month will earn reward at 0.35p per pound (worse than a cashback card).
What about a Curve Metal card?
This will cost you £150 a year (if you use the annual payment option) and provide a number of other benefits which you may or may not want. Since this offers unlimited Amex spend, you will need to spend £15,000 to break even. However, as you might know there is a limit of £50k annual spend on a Curve card. Therefore, that is £35k which you can earn unencumbered points on (35,000 points will be approximately £350). It’s a shame that there is an annual limit, because at this point you are making more money than you could via other non-Amex cards.
When does it make sense?
It might make sense if you value the other rewards of the black/metal card highly.
It makes sense if you have the metal card and are spending significantly more than £22.5k a year on it (remember that because a 0.5% cashback card would make £75 on £15k and an Amex makes roughly £75 on £7.5k, you will need to spend at least £22.5K before you have made more money than using a free cashback card would have made you, and it’s the extra £12.5k on top of this in which you would have a higher earning rate).
It also makes sense if it helps you to reach a spending goal on the American Express which you otherwise wouldn’t reach. For example: to get a 2-for-1 voucher.
Finally, it might makes sense if you are able to put any large bills through with it (HMRC Self Assessment, VAT, etc), which otherwise wouldn’t accept American Express.