Hey James,
In the examples you mentioned, what I was exploring was a mechanism of attack by which the attacker could steal user A's key and use that key to send a transaction with the maximum possible fee. User B would still receive some funds (probably), but if the fee could be large, the attacker would either do a lot of damage to user B (griefing) or could make an agreement with a miner to give back some of the large fee (theft).
But as for use cases, the proposal mentions
a number of use cases and most overlap with the
use cases of op_ctv (Jeremy Rubin's website for op_ctv has a lot of good details, most of which are also relevant to op_cd). The use case I'm most interested in is wallet vaults. This opcode can be used to create a wallet vault where the user only needs to use, for example, 1 key to spend funds, but the attacker must steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault like this vs a normal 2-of-2 multisig wallet are that not only does an attacker have to steal both keys (same level of security), but also the user can lose one key and still recover their funds (better redundancy) and also that generally the user doesn't need to access their second key - so that can remain in a much more secure location (which would also probably make that key harder to steal). The only time the second key only comes into play if one key is stolen and the attacker attempts to send a transaction. At that point, the user would go find and use his second key (along with the first) to send a revoke transaction to prevent the attacker from stealing their funds. This is somewhat akin to a lightning watchtower scenario, where your wallet would watch the chain and alert you about an unexpected transaction, at which point you'd manually do a revoke (vs a watchtower's automated response). You might be interested in taking a look at
this wallet vault design that uses OP_CD or even
my full vision of the wallet vault I want to be able to create.
With a covenant opcode like this, its possible to create very usable and accessible but highly secure wallets that can allow normal people to hold self custody of their keys without fear of loss or theft and without the hassle of a lot of safe deposit boxes (or other secure seed storage locations).
Cheers,
BT