There is a wide gap between a straight ahead quest and Martin's scattergun approach. I've read the first three books, they're great and they do provide a lot of necessary and valuable context. I've also literally felt like I've read chapters that were utterly worthless.
Martin himself has admitted he started with a three book idea but gets "fascinated" with characters along the way and stops to admire them. Characters that have a minuscule chance of doing anything more than eating pages. It also has become obvious (and people tell me it becomes MUCH worse come book 4) that more and more time is spent on these cute deadends at the expense of the end-game.
Cute deadends would make great ancillary books. As part of the main canon they tend to stretch out what really matters and pretty soon rather than walking the straight path you forgot you headed the wrong direction for a year or two.
It was worse in book 4... took a while for it to develop. Book 5 on the other hand was much more too the point as it tied all of that up. I'm really hoping Winds of Winter concentrates more on the main characters and sticks more to action.