Well, for a "so called" professional modeller it may be worth using these models/styles/materials whatnot... You can always keep it in your final product like renders, animations etc. that you have already "sold" to the clients.
And do not forget that most (well, "all") of these models are very high quality. As low poly as possible without to much visual compromise, all faces are correctly oriented, no "litter geometry" inside and all those nuances that make a huge difference between these and an average Warehouse model where you must start with cleaning up first and if it is a really crappy model, you may spend more time on it than if you started from scratch. And a "professional" modeller is professional because he is modelling for money - and time is money.
So all in all, there is a circle of modellers to whom it may be worth this subscription system.
(And let's not scold FF too much as we have a bunch of nice people here who are also FF artists)
😄