Ecash are promissory notes for sats. Address locking is a cool security feature for protecting against ecash scrapers malware/bots. This alone is reason for them. Address locking ecash does not improve counterparty risk from the mint as the operator (or a signing quorum) can *always* rug all the sats from the mint.

I think the greatest advantage of Fedimint at this moment is that a multisig quorum provides a path for skirting regulations. No single key holder could legally be considered the custodian.