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I think there's a bit more to it than that, even granting the optical reasons why f/7 is preferable (and that after all was what Hartung recommended).
An f/5 tube is not only cheaper to manufacture, it is also - in apertures of 10 and 12 inches - far more portable than f/7. A 12 inch f/5 isn't too hard to move in a car. The same aperture at f/7 would be nearly two feet longer, and a very different proposition. This is a real consideration if you do not have a permanent observatory. Thus, f/5 is likely to have a bigger market than f/7.
As in most things, there are tradeoffs. The coma in my GSO 12 inch f/5 is not oppressive. It's an acceptable price to pay for the lightgathering power. And, incidentally, for paying about one fifth of what one would have to find for a 12 inch Schmidt-Cassegrain.
jim
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