I've always thought it was necessary to get permission if you are modelling the piece for your portfolio. There are a few peoples concepts that I found on CGhub that I REALLY want to model for my portfolio, but they never respond to me.
Well its all about context really. And in this context, it's not at all about legalese, its about the usual practices in this field. I think the OP is, above all, asking for the point of view of people having first-hand experience of the situation. And no, it's not all about the network of artists getting pissed at someone…