OK, I looked into the PyMVPA codebase, but I could not see more than a handful places where this method is used. I really feel this is an awkward part of the API -- the main theme is "doing something with a dataset yields a new one".

I'd want to veto a further extention of this function. I'll try to create a separate PR that deprecates this method, and determines whether it can be fully replaced by h/vstack().