I would say fragmentation is avoided where the writer can keep them focused on the main plot, whatever that might be. I would say that I think this is easier to do over the course of a single book, than over the course of a series. Take "Magician" by Feist, as you have mentioned him. A book with a number of various plots lines, and though there is a clear main character in Pug, there are two other main PoVs, and a couple of other more minor PoV incidences. Yet it all remains strongly focused - within a single book - on a particular plot-line and all has to be brought together by the final page. That I am sure greatly helped Fiest. In comparison both Jordan and Martin can always carry things over into more books, and I feel that they have rather let some of the subplots run away from them - especially Jordan. Looking back it is one of the oustanding things about the GAP series, is how tight it is compared to many multiple POV books.
I think the problem, as such, is that most books in series do not have their very adequate conclusions in their own right. The Riftwar is an exception to this, as Magician, Silverthorn, and a darkness at Sethanon are all really single novels set sequentially, rather than a trilogy per se. And although most series novels have a sort of climax, there is usually not conclusion. I am thinking of the ending of Chaos and ORder, book 4 of the GAP series, where there is a most dramatic climax - but absolutely no conclusion. Now, at that particular point Stephon Donaldson has things so well focused it does not matter, everything is already hurtling towards the conclusion that is This Day All Gods Die. But where I stopped reading the Wheel of Time (book 9) no end is in sight. Does that make sense?
Alongside this discussion of multiple points of view is one of multiple points of time, which I think is very similar. Take the example of the Deverry/Westwind/Dragon Cycle books of Katherine Kerr, written higgledy-piggledy over about four or five centuries in her fantasy world. While 'the present' is the main storyline, we keep going back into the past to tell further stories there, building up an image of history and of the past, yet that is focused around the fate of a group of particular souls that keeps it all together.