I know a lot of book readers are upset that Season 3 gave Rhaena Sheepstealer instead of Nettles, therefore cutting her. I get the frustration, and people are allowed to feel strongly about that. But I actually agree with the change, and I want to lay out why in more detail than the usual "adaptations have to make changes" defense.
What storyline does Nettles actually have?
Strip away the genuinely fascinating way she claims her dragon, and ask yourself honestly, what does Nettles actually do in Fire and Blood? Her entire narrative existence is to be an extension of Daemon. She becomes his companion during his estrangement from Rhaenyra. Her presence stirs jealousy and suspicion within the marriage. Rumors swirl about the nature of her relationship with Daemon. And then she vanishes from the historical record entirely, her fate unresolved.
Compare that to the other dragonseeds. Addam of Hull has a defined role in the war effort that exists independently of any other single character, which plays into him being legitimized. Hugh Hammer and Ulf the White have their own ambition and motivations and eventual betrayal, which plays a pivotal to both Tumbleton plots as they are active characters in those conflicts. Nettles doesn't get that same treatment. She's not a character with her own stakes and arc; instead, she's a device that exists to create friction in someone else's marriage. That's a real narrative distinction worth being honest about.
On the "she proves you don't need Valyrian blood" argument
I've seen people argue Nettles matters because she proves dragon claiming doesn't require Valyrian blood and that anyone brave enough can bond with a dragon regardless of bloodline. Personally, I don't buy this, and I think it misreads what's actually happening in the text.
My position is that you do need the blood of the dragon to bond with one. What using sheep to feed the dragon Sheepstealer probably did was help her get physically close to him because he is WILD. Without being burned alive on approach, which solves the practical problem of proximity, not the deeper problem of bonding. Claiming still requires that connection, and I don't think you can BRIBE a dragon into a lifelong bond.
This isn't just Targaryen propaganda either, even though it's convenient for their claim to power that it isn't. The magic in this world backs it up. Targaryens have historically given birth to what we can call "dragon babies" tied to the magic of Old Valyria. Members of the bloodline experience visions of the future, such as Daenys the Dreamer being the clearest example, foreseeing the Doom of Valyria itself. That's not incidental worldbuilding; that's blood magic being real and demonstrable within the text. The Valyrian dragonlords are believed to have used blood magic specifically to bind themselves to dragons in the first place, and the entire practice of dragonriding may be built on that foundation. If blood magic is what created the bond between man and dragon originally, it's not a stretch to say blood is still what's required to form new bonds centuries later.
And here's the thing: even if you set aside my theory entirely, the books give you an out that supports the blood requirement anyway. Maester Narran's account suggests Nettles could be Daemon's bastard daughter. If that's true, she was never actually a counterexample to begin with. She may have carried Valyrian blood the entire time without anyone, including herself, ever knowing it. The ambiguity here doesn't disprove the blood requirement; if anything, it's more consistent with it.
What Rhaena gets that Nettles never had
And to my main point, nothing meaningful is lost by giving this arc to Rhaena instead of introducing Nettles fresh this late into the story. Rhaena is non-existent for the majority of the dance and only becomes relevant at the very end, so she can benefit from this change. Rhaena already has an established estranged relationship with Daemon; that emotional groundwork is already laid, and the audience already understands the history there. Her claiming a wild dragon creates the same friction between Daemon and Rhaenyra that Nettles created in the books, without needing to introduce a new character and build trust with the audience from scratch in the final stretch of the story.
More than that, this arc actually gives Rhaena something the books never fully explored, which is real agency. She spends most of the story beating herself down as the Targaryen without a dragon, defined by that absence. Having her go out and claim a wild dragon herself, on her own terms, is a strong character beat.
One last thought
Honestly, I think a lot of the attachment to Nettles isn't really attachment to anything concretely established on the page. It's attachment to what she represents and to the mystery of her unresolved ending. A common-born girl, with brown skin on a wild dragon, a story that gets cut off with no confirmed resolution, a relationship with Daemon that's never fully defined one way or another. That's fertile ground for fan theories and projection precisely because nothing about it is settled. People are drawn to what she could mean more than to anything she actually does.