If he wanted to take advantage of being paid by the book, there are easier ways to do it than writing installments so massive they run the risk of being impossible to bind. I don't think Martin is afraid to minimize the importance of characters if he has nothing to write for them; that's why there are only four Bran chapters in Storm
and only three chapters each in Feast
for Sansa/Alayne and Arya. (When it was one big book, Feast
was also going to have only four chapters of Tyrion.) There's clearly some purpose behind Arya's and then Brienne's sojourns in the riverlands. Whether that purpose warrants the pages spent on it... well, like I said, I'd have to get back to you on that once I reread the books.