Why are doubled polarization filters as continuous ND not popular?

Question

One thing I was always wondering when it comes to ND filters is why there's need for such a large amount of different strengths, when two linear polarization filters would do the trick just fine. Assuming a light loss of 1/sqrt(2) per filter, one could easily create a ND "1 to infinity" filter. Why is that not in everyone's camera bag?

Unfortunately, all the filter companies offer only LCPL (linear, then circular), so I can't simply make one myself. Apart from cases where I do not want to take polarized light into account while having a ND filter (architecture photography maybe), I can't think of a reason not to do this.

Edit: More specifically, I'd like to know why there's nothing like this at all, or how I could get my hands on such a filter.

Answer

Does Singh-Ray's VariND filter not count?

Note, though, that even in their own product advertising, they point out the fundamental problem with this filter arrangement: it's still a polarizer, so it's still going to give unsatisfactory results on skies when used with a wide-angle lens. And there's that little thickness problem as well; vignetting is going to be an issue on wide lenses or on lenses that don't have a lot of wiggle room between the front element's edge and the filter thread.

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