Let's say you're programming in an object oriented language, and you have a parent class which contains some, err, poorly developed code. If you make a child class which extends the parent class, you will bring the poor code along.
Parent / child = descent. But it's still design in that case.
Just stirring the pot.
Cheers!
I understand that common descent is compatible with ID, but I thought I was responding to an argument against common descent (I could be mistaken)