I think it is possible that he was trying to avoid some of the problems that RJ ran into after book 6 by allowing too many separate story arcs to unflod simultaneously. It is true that there is something lost in all of this hacking up of the timeline. But perhaps he felt it was an unacceptable risk to have a book that pushes each story ark the same amount and winds up having not a thing happen in any of them.

I know that some people were able to enjoy the great amount of subtlety happening in books 8-10 even while the plot progressed so little. But most people, myself included, were simply bored to tears and frustrated. If TGS had come out and in the end nothing had been resolved, ie Egwene was still a prisoner in the tower, Rand was still angry and dark, Mat had not gotten Moiraine out of the tower, Lan is still just a solitary man traveling the blight, etc... then perhaps too much of the fanbase would have checked out permanently now that the old author had passed on and the new one was both a less brilliant writer AND just as boring..

I realize I have oversimplified things by saying that there would be nothing resolved. But perhaps Brandon (and Harriet) decided that an inherent flaw of the series as RJ was writing it was that in order to maintain the depth and subtlety, the plot had to progress far too slowly for most readers' tastes. Assuming that Tor would not let TGS be 1500 pages, perhaps Brandon felt he either had to obliterate the continuity in order to progress at least some characters' arcs, or do what RJ did and progress everybody's arcs too little for most people's tastes.

I for one enjoyed TGS more than I enjoyed books 7-11, and by a very large margin. If I have to choose between a hacked up timeline or another set of story arcs that drag on over multiple books (like Perrin rescuing Faile, Elayne very slowly becoming Queen, Egwene and her army plodding toward and attacking Tar Valon, Mat escaping with Tuon and eventually getting her back to her people, etc...)

Those storylines all developed so slowly over so many books, that many people didn't care anymore if RJ was subtle, deep, and layered. They just wanted him to get on with the story. RJ's style of writing exacts too high a price on plot progression when there are so many story arcs going on. He let the number of story lines get out of hand and the result of maintaining his writing standard while dealing with so many different story lines was resoundingly bad for perhaps the majority of the fan base.

Brandon and Harriet may have decided that the hacked up timeline was the better of two evils. They may have been right.