We need it.  Generically, we don't know which attributes have been attached from the `model.__init__` signature.
This should also automatically catch weights in WLS, rho in GLSAR, ...
I got started with this a few days ago because I added an attribute in a Poisson subclass that broke llnul called by summary(). I fixed that in a different way, but it's going to happen all the time.

The alternative to a generic solution, requires lot's of code that handles special cases.