Definitely, but even then they added a few brothel scenes with Ros and Littlefingers (it's been a while, but I remember at least one appearance by Ros in KL in season 1). These scenes gave insight into the sort of man and player Littlefingers is, even though literally it was about whoring. I'm not really arguing they made a good decision to add her, so much as observing the writing tricks behind having a character like Ros. It's also quite obvious those scenes are also there to meet the target amount of nudity per show, one's not exclusive of the other, especially not with the usual standards of script writing of HBO dramas.

The one gain I already see in having these interactions with Ros (and the fact she's from Winterfell merely a nice touch), teaching her to be a better whore, is that it puts an interesting twist on what he will be doing with Sansa at the Eyrie later on (you had that aspect in the books, remembering LF had been a pimp/brothel owner, but his interaction with Ros makes this more explicit/TV friendly). It would be even stronger if later they replace Alayaya with Ros and have Littlefingers pull her strings (she was the other whore character I had in mind but forgot the name of, the one Roland mentions in his post). I have the same impression he has they intend to have this part played by Ros, who will be sufficiently well established by then (and it could mean the secret arrangements may be between Tyrion and Littlefingers - in Cersei's back - instead Tyrion and Chataya, or maybe that Littlefingers will be pulling the strings of Ros without Tyrion knowing, for instance sending her to another brothel) - both would work as devices to increase Littlefingers's spiderweb, or rather, to make his plans/involvement more explicit for the TV audience.

In short, they seem to see Ros in terms as a kind of sounding board/sidekick - a character who's not very significant and won't ever be much more significant, but whose presence in the show allows Littlefingers to throw the occasional exposition nugget at the audience, in scenes without any of his adversaries present. They can keep her naked most of the time too, so it doesn't hurt.

Which seems like already a strong argument against her existence, since it means that viewers don't distinguish between the show's whores and don't care about her character build up

Well, I did notice her and her reappearances as I watched season 1 when it was on the air (and remembered immediately who she was when I read the posts above, though I've not started to watch s2 yet), so I expect a good chunk of the audience did too.

I wouldn't expect them to waste much screen time on Ros, just enough so eventually she'll be recognizable when they need her for something else than providing an interlocutor for LF.

There are other examples of the device in the series, for instance adding the scene between the King's younger brother and his lover. It's all tricks you have to use when adapting a complex story, told via POVs in the original novel.

Given his solid backgroud in TV writing and his understanding of the medium, I wouldn't be surprised if Martin has even made a few suggestions of his own, even for departures like the inclusion of a character like Ros, who was there from the first episodes (her first purpose was to give early hints about the real feelings of Theon, which meant having him interact with someone outside the Stark circles. Again, she was used as a passive sounding board/sidekick. But to bring her to KL required quite a bit of forethought. Could be the scriptwriters, who after all know the series well, or it could well be Martin, whose knowledge of his own plot mechanics is forcibly even better, and who might be more ruthless than the adaptators about making changes. Some novelists are very protective, others are among the most ruthless adaptators - John Irving for e.g. - none of the adaptations of his novels has departered further from the book than the one he wrote himself, cutting characters, merging some, adding some, changing their roles, writing whole storylines differently etc.).