MLS works great for 1-1 chat. There’s slightly more cryptographic overhead than a signal double ratchet but really it’s almost negligible. And MLS makes turning a 1-1 chat into a group trivial, which is something signal protocol can’t do efficiently without Signals servers in the middle.

Double ratchet rotates message keys after each message, so previous consecutive messages from the same author become undecryptable, when the previous message key is discarded after decryption. It's a small difference, but it's good for disappearing messages.