Free use can be boring. It's generally the execution where it lives or dies. How people around the character react to it, the way relationships change, the situations the character finds themselves in.
Seconding @Zeebop ’s point; any genre/ trope can be boring if the author doesn’t explore the area well. I don’t even much care for that particular subgenre but a well-enough written story can get me intrigued!
Most FreeUse has bad worldbuilding and is too one-sided with just guys using women and nothing else. I have thought about a more interesting FreeUse setting, but most of the fans of the kink don't care for that.
I will be honest. The dynamics around free use get soooo interesting if you stop and think about it. Who gets who, are there any laws, can you get sued because you "used" someone in a way that offends someone else?
I don't fault anyone for just enjoying a simple "the girl gets fucked" story tho.
The taboo is part of what makes sex stimulating. Just the physical act itself isn’t all that interesting. Free Use takes away a core part of the appeal of doing something forbidden. When everyone does it, it’s no longer as interesting.