Using XEP-198 over BOSH adds no value.
I tend to disagree here. The following (from XEP-0198) is true for BOSH as well:
it is desirable to quickly resume the former stream rather than complete the tedious process of stream establishment, roster retrieval, and presence broadcast.
Also, what if a client connects with BOSH, then disconnects due to network failure, but the server doesn't recognizes it fast enough (usually only after the polling interval has exceeded, which is usually 60 seconds). The BOSH connection manager would return the HTTP response with some messages, but it wouldn't receive the user and would probably get lost.
If SM would have been negotiated, the client could resume the old stream, inform the server about its stanza count and the server would deliver them.