As far as I see it storytelling comes first. If themes get in the way of a story they need to go. If you have too many characters and themes it becomes unwieldy.
What I mean is that they could have taken all the key events of both books and fit it into one. Quentyn dies. He never interacts with the characters in a meaningful way and so ultimately serves no point than to make a statement. The story gains nothing having him, and as far as I see it if you don't serve a purpose in the story you're dead weight. "The theme comes first" is shit writing.
And no Martin himself implied that the battles were cut for time; he fully meant to include them at the end of book 5. It was his inability to delegate. In any case having too much is probably why he has yet to release book 6.
The important element is that things happen. In the books it's "characters spend time brooding, they go to a place, they talk, they leave"; contrast with the first three books where major events happen.
The show has problems but it's actually a lot more streamlined. Important things happen in a "mostly" believable way; important characters are established and developed. Story events occur and characters evolve.
As an aside; My dad loved the first three books and felt the show inferior....until he read books 4 and 5 which killed his enjoyment, since he saw them as piffling twaddle that dragged out the story.
As Jon of the living dead says
Jon of the Living Dead wrote: Well, first a disclaimer: I don't think Quentyn the character was unnecessary, I think Quentyn the POV was. I do think his death will have important consequences
Now, 4 chapters is a lot, actually. It's more than Asha had in aDwD, more than Melisandre, more than Jon Connington, more than Victarion, more than Arianne ever got, all of them arguably more interesting and more instrumental for the future plot than Quentyn
Also, he's a Prince of Dorne, and? So was Oberyn and we never got his POV (which would have been great)
Yeah, Martin uses Quentyn to deconstruct the classic archetype of the hero's journey...for like the tenth time. We had that with Ned, Robb, Oberyn, the Brotherhood, etc, so it's hardly new ground. It's repetitive.
Finally, let's not kid ourselves: the guy was likeable, but his characterization was paper-thin for a POV. He could have worked great as a regular character, just not as a POV
He brings nothing new, he hogs time from Victarion Arianne and others, and he has the characterization of a plank of wood
Emmet Booth is an idiot; he's like those jackasses who defend Sam Harris and ignore that he's a genocidal monster. GRRM should have ditched Quentyn entirely