Also lower transaction costs and liquidity provided by third parties, but that benefits the service more than the users.
I suppose the next question is what fees are Mercury going to be taking for running this service? They don't actually have any transaction costs as far as I can see, since the person depositing the coins to the split key address pays the transaction fee there, and the person ultimately withdrawing the coins will pay the transaction fee on that end. But they will obviously need to pay for running and maintenance of there servers. And when is that fee taken? There's a privacy implication there too if a deposit or withdrawal transaction also has to pay a small amount to an address which can be identified as belonging to Mercury.
Each time a statecoin is transferred, the backup transaction used in the event the SE becomes uncooperative has the nLockTime value decline by one.
There is nothing saying the nLockTime value must decrease by 1, and indeed, doing so leaves it open for previous owners to steal transactions which are RBF enabled.
Additionally, the transfer can incorporate the cooperative signing of a new backup transaction paying to an address controlled by Owner 2 which can be confirmed after an nLocktime block height, which is shortened (by an accepted confirmation interval) from the previous owners backup transaction nLocktime.