1- “I use catart-mubu, Catoracle, CataRT Standalone and OMax, primarily for improvisation. Are there reasons I should learn Open Music?”
Well the answer to your question is somehow relative to what you are up to. OM is basically a non-realtime environment.
2-“Is there a way to harness bach to catart-mubu achieve some of the same purposes as Open Music? Has anyone tried this? Please share your experience and perhaps your patches.”
bach externals are mostly a port of some OM musical editors. They have been used with other max implemented programs such as Orchids. So maybe you can find a way to implement them. You should address this issue to the max community.