I think it's best if a show does diverge from the books it's based on once it establishes its own identity. For one thing, you never know which actors will break out from the pack or turn out to have chemistry. If the heroine's love interest in the book turns out to be played by an actor she has no chemistry with at all, while the minor supporting character turns out to have great chemistry with her and steals every scene he's in, it would be wrong not to have the heroine leave the book love interest and hook up with the supporting character. The show has to be its own entity and do what best serves it as a show.
In theory I agree with this..... having read all of GOT long before the tv show and having watched the series, I don't think that could work for this show as well. I don't mind the changes they're doing, or what they're adding, in fact I love the show in its own right, but only because they're still not changing the overall plotline or where its headed and I don't think they can. In a series like this, where who people fall in love with are critical to what happens books later, couples can't really be changed up on a whim. You can't really change the overall direction of the book's plotlines and still have things work out. That being said - I do love the show, changes and all, and don't quibble about them. There were some things that surprised me, but I liked them.