Just to add to my comment [here](https://github.com/astropy/specutils/pull/71/files#r13293354) another reason inheriting from PolynomialBase might not be the right idea here is that that class has no notion of a piecewise-defined function.  Perhaps later it will made sense to add a generic notion of piecewise models which, combined with generic polynomial models, could be used to define splines.  But for now it makes more sense to treat it as a special case (we do have a few other piecewise models like the box model which implements a step function--again, a special case for now).