Well, I can’t speak on behalf of Curve, but their support team has been very helpful in trying to help me verify my Coinbase card. But as Coinbase are actively blocking Curve, there isn’t much that can be done.
The point reamins though, the Coinbase card is a VISA Debit, and the underlying currency is not crypto, but Pound sterling. If for whatever reason someone would want to get Coinbase, and only keep GBP/EUR/USD in their wallet, then they would be able to use the Coinbase card for transactions with those currencies.
With your reasoning Revolut should not be allowed with Curve as well. Revolut is in a way identical to Coinbase card. Both are “regular” Mastercard or VISA debit cards (or prepaid cards), and their underlying currencies are both Pound sterling. Both cards (Revolut and Coincase card) can “use” cryptocurrencies for transactions. But the underlying currency remains GBP.
Again, my understanding is that Curve wouldn’t have a problem with users adding Coinbase cards. The Curve support team being very supportive in my case trying to get my card to work seems to confirm that.
Using Curve to buy cryptocurrencies (whether it is on Coinbase or any other website) is another matter of course.