The SBE command no longer works for simple queries after the changes we made in the SBE plan cache and auto-parameterization project. I'd argue that we should either continue to maintain it, or just delete it. This ticket suggests the latter approach.
As far as I know, the SBE test-only command was used for testing early in the development of the SBE engine but is not used any longer. It is not well tested and so we would generally not notice if we break it. I suspect that its parser and the SBE plan debug strings have already drifted out of sync. It also imposes some maintenance cost on the Query Execution team, as it can be executed by jstestfuzz and by an integration test (sbe_cmd.js).