The BinaryMarshaler API returns a freshly allocated byte slice. This makes it unadvisable to implement for high performance APIs, or even APIs that could happen to be used in high performance scenarios.

If it had an append-like API (MarshalBinary([]byte) ([]byte, error)) then also a lot of high performance and internal parsing operations could be BinaryMarshaler implementations, allowing more standard interface reuse.

The usability is not much worse, as nil can be passed to get the exact same behavior, and there's precedent in hash.Hash.Sum.