"Sexposition" (a term coined by Myles McNutt) or else some straight up "Hey! Boobs!" (an idea put forward by Mo Ryan). Ros has been involved in some of Game of Thrones' least necessary nudity. She was at the heart of the show's most controversial example of sexposition, namely the whole 'Littlefinger Presents: King's Landing Idol - Brothel Edition!!!' ordeal. It seems like she exists for the express purpose of filling HBO's episodic boob quota. Why even bother having her around? Boobs are fun and all, but honestly, I'm trying to enjoy me some Game of Thrones. If I wanted to watch softcore porn, I would find some softcore porn to watch. Not to mention the awkwardness of watching this show with your friends and their parents. That's always a good time.
Regardless, there's certainly a larger discussion about the necessity of nudity in television to be had; however, due to a lack of space and time constraints, in short, I stand firmly on the side that favours nudity for the sake of realism. If main characters are having sex, then nudity should be fine. Tyrion and Shae come to mind, as does this episode's Melisandre/Stannis scene. Also, if you enter a brothel, nudity should probably be happening. Should we enjoy 30 seconds of sexy times before making our way over to the plot? Probably not (although I'd have to look at each situation on a case-by-case basis; setting the mood is sometimes both interesting and informative).
The rather long-winded point I'm trying to make is that we don't actually hate Ros personally. We hate unnecessary nudity. Up until now, she has largely been involved in the worst of it; however, her two scenes so far this season have turned that idea completely on its head. She hasn't yet appeared naked, nor has she had sex on screen. She was present during a Hey! Boobs! moment last episode, but I would argue that it was a somewhat hyperbolic use of nudity to set the mood for imminent baby brothel murderings. It also served to show us that she had become the teacher (in direct contrast to the aforementioned Littlefinger training scene).
While some more Littlefinger/Varys confrontations would have been exciting, I would argue that this scene revealed a lot more about Littlefinger's character than another of those encounters would have offered at present. He tends to be coy and playful while confronting the other series regulars. It was only through a tertiary character like Ros that we could truly see the depths of his callousness. The scene could definitely have used some shortening here and there, but all in all, I was quite pleased with how it turned out. Any complaints about nudity lie with Littlefinger, since this was HIS scene, and not with Ros.
Conclusion: Don't shoot the messenger!
Regardless, I believe that these fears are completely unfounded (at least as of the the start of the second season). The most important point that I think I can make is that Ros seems to be taking on the role of Chataya. As such, we can't actually argue that she isn't someone from the books. We knew that, with a cast of several hundred characters, we were going to have to make some compromises for the sake of television storytelling. We knew that certain minor players would be combined into a single character and that others would be dropped entirely. The only real complaint that I would levy in the direction of the casting directors is that Ros becoming Chataya effectively white washes the character (Chataya was black in the novels). I won't get into that now, but issues surrounding diversity casting are absolutely worth discussing and are of concern.
You might argue that we don't actually need any sex-with-prostitutes scenes at all, but I think that placing characters in vulnerable positions is a great way to find out more about them. Grand Maester Pycelle was revealed to be much less of a senile old man than we had been led to believe, Theon has control issues, Littlefinger has too many issues to name, etc.
Conclusion: Ros is a useful storytelling tool.
A final point I'd like to make is that we should really calm down about the existence of new characters. If they have nothing to offer us, then absolutely, hate away... but the initial gut reaction causing us to scream 'SHE ISN'T PART OF THE CANON SO GET HER OFF MY SCREEN' is something that we all ought to fight. This isn't the novel, nor could it ever be a direct adaptation of the novel.
Conclusion: Let's see what television has to offer!