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I agree with this approach.  Categories should only be created when they are expected to have reasonable traffic.

A category with only chirping crickets is an anti-advertisement.  Take a look at the Photography and Outdoors sites.  They have lots of nearly unused categories, which I think makes those sites look even more dead than they already are.

Your idea of a special tag seems like a good solution for now.  If by some miracle we get flooded with design review questions, it will be easy to find the existing ones so that they can be moved to a new category.