I agree.  I have some thoughts on better generalizing how polynomial models work (Nadia and I have both done some work on that but it doesn't yet quite fit in).  In the meantime, however, this implementation for B-splines doesn't actually gain anything from inheriting from PolynomialBase.  

I think the most important thing for you now is having something that works for the cases you need it for.  We can worry about class hierarchy cathedrals later.