While I think this is a good idea for smaller data sets, it just isn't feasible for larger data sets that require indices. Having every combination of columns as indices is just hell for the RDBMS. When you are faceting with a large dataset, it's better to use something like ElasticSearch or Sphinx.
I don't think anyone was implying that you would expose this for every combination of columns, only the ones you were allowing people to query on. Presumably, you've indexed the fields you want people to be able to scope results to.
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While I think this is a good idea for smaller data sets, it just isn't feasible for larger data sets that require indices. Having every combination of columns as indices is just hell for the RDBMS. When you are faceting with a large dataset, it's better to use something like ElasticSearch or Sphinx.
I don't think anyone was implying that you would expose this for every combination of columns, only the ones you were allowing people to query on. Presumably, you've indexed the fields you want people to be able to scope results to.