The initial context of this package was WCS support and this particular model may not be applicable to astrophysical models. The use case it covers is a detector with a distortion described with more than one model, e.g. one component is a polynomial model and another component is a filter dependent displacement represented as a lookup table. So 'x' and 'y' are coordinates in slightly different coordinate systems and in this case the above statement represents the mathematical operation. As an example, in astropy.wcs this is represented through sip_pix2foc() and p4_pix2foc() methods. The disadvantage there is that this is hardwired and other models cannot be used. The aim here is to make it more general and allow other types of mathematical (unitless) models.
When units are added to parameters I think there will be an optional None value for parameters of mathematical models.
But I see your point about astrophysical models. May be there needs to be a check for the type of models added in this way and a rejection mechanism if any of them are astrophysical models.
Or would it be better to rename this class to something which indicates its intended use?