It has every relevance to what you said. You said How is this even a question to book-readers? The answer is that book readers know that Theon disappears for 2 books. If the show reflects that, its not "doing away with the storyline altogether" or "unnecessarily delaying it". It's following the storyline and building the narrative tension of "what has happened to Theon?"
Now, if the TV show wants to bring forward Theon/Reek by 2 or 3 seasons, that's their perogative.