Well what did you just do by putting a Paracorr in after the secondary? You just did for a parabolic mirror that a Jones Bird does for a spherical mirror. Why don't all parabolic Newtonians come with a coma corrector built into the focuser? I guess the biggest problem with the Jones Bird is simply collimation. If the elements were well made and easily removable and repeatably replaced for collimation there would be a lot more of these around.
I think it must be more than collimation. Bird-Jones scopes have a pretty bad reputation - even middle-end ones from back in the day usually did, and you'd think at least some of them would have been collimated right. Mak-newts and Schmitd-newts, which do the same thing with a full corrector plate, are far better regarded, despite having all of the downsides associated with a corrector plate up front (dew magnet, heavy, slow cooling, more glass).
My offhand guess is that doing the correction with a small post-mirror optic stresses the lens far more than doing it with a big pre-mirror corrector plate, so it's extremely hard to make a good lens (probably, it's the chromatic aberration). That would explain why cheap ones are so bad, the middle-end ones are not great, but some people have positive memories of high-end ones. It would also explain why the design is dead for anything but the lowest level of scope - if it costs more to make the lens than to make a corrector plate or a parabolic mirror and a coma corrector, there's really no point.