Cumulative effects are absolutely an important part of doing good project assessment. Could it be most effectively done in a broader land use planning? I think the answer is yes. Do I think that regional and strategic planning are good ideas? Absolutely.
In a project-specific context, the difficulty is knowing information. If you broaden things out to what might be possible or even conceivable, how is a project proponent able to come forward with credible information about speculative things? There is a practical problem with it. Conceptually, it's a good idea.