Generic models don't have restrictions of editorial uses (editorial licenses).
In practice many 3D models are made without consent of the original copyrights owners, brand holders, designers. (in theory this is copyright infringement)
Full ownership cannot be claimed on this sorts of models by that creator. The original copyrights owners have the right to come up with different term of use (ad any time).
Someone hoe purchases these kinds of models need to be careful how they are going to use the models. (risk of getting sued by original copyrights owners if they do not agree the use of them in film/game etc.).
Generic models don't have these risks attached.
I do not have statistics but my perception is that generic models are preferred in most situations. (ones had a client asking me to re model parts of their Audi A8 stock model, to turn it into a generic model for them)