Quentyn isn't pointless. We can debate whether he's worth four POV chapters, but his role in the narrative is vital in a couple different respects.
Aegon has been foreshadowed since A Clash of Kings
; he's not a last-minute invention or a response to the structural issues in books three, four, and five. Daenerys is taking forever to get to Westeros for the same reason it took Sansa forever to do anything but get abused and cry about it, or Arya forever to do anything but get abused and seethe over it: because the narrative bloat around other characters turned one book's worth of near-stasis into two or three.
I know most people love A Storm of Swords
, but I think structurally speaking it's as flawed as the two later books, and is easily the worst in the series for coincidence and contrivance in service to the plot. A fast-paced ending doesn't make up for Arya and early Jaime chapters that are much worse than the Brienne stuff from Feast
for telling us things we already know about the horrors of war, or Davos chapters that amount to him sitting in a cell and being told the plot. It's usually suggested that the problem with the series was Martin abandoning the five-year gap, but the bigger mistake was inventing it in the first place when he restructured the series while writing A Clash of Kings
. If book three had ended where the first book in the original plan for a trilogy did, with the Red Wedding, it would have been easier to move some of the setup elements from Feast
alongside the eventful stuff from the last three hundred pages of Storm
; it's all basically simultaneous anyway. Storm
wouldn't be as much a fan favorite in that case, but the series as a whole would probably be a lot less messy.