Without DeclHandler, I don't think we have any way to know which attributes are IDs, among other things. I believe that losing that info could cause some XPaths to malfunction. So I think this requirement is deliberate.
It might be possible to record the fact that we were unable to register the DeclHandler, and only report it as a problem if we try to do something which requires that support. That's probably worth recording as a request-for-enhancement.
