Here's the thing: Ros does appear in the books. In the first novel, she is thoughtfully named "The Red-Headed Whore" by old Georgie Boy. While that name has a certain ring to it, "Ros" might be a tad bit more humanizing. And that's important, even for a character who only really serves the story by getting naked and catching a crossbow bolt in the chest.
That said, on this issue, generally, you have to concede that the book readers have a point. As of the end of season three, the TV series had added almost a dozen sex workers that R.R. hadn't thought up. Weiss and Benioff went a little crazy when they realized that Westerosi geography lessons were a lot easier to swallow when given during threesomes. To the show's credit, they've walked back the courtesan-driven sexposition a bit recently.