The martells do have a claim to the main historical event of the books, Rhaegars wife being a one and all, but damn...I can't care about them. They want what they dont have, what dany has, which is what nearly everyone else wants, so spending time on them was quite tedious reading for me. The Greyjoys, as a clan, achieve the same thing, as do all of the clans of people from essos clamoring for danys attention, as well as nearly all of the other main forces, once they get wind of the events in essos.
Also, the women of the family fall into the ludicrous archetypes that Martin has formed for nearly all the females in his saga....slut, insane bat shit crazy, pretty and powerless, or "wannabe a man so i can wield power". Again, this doesnt make them special, since just about every other female, besides arya and dany are previous examples. I dont get why we need chapter after chapter on them.
Its just so ponderous of a story he gives us. I realize that part of what he is doing is the grim nature of the actual "game",that people will just die or disappear (or re-appear) because he's trying to stay true to the violent nature of his story. At the same time, I get much more enjoyment reading about the course of the various stark children, the events around the wall, anything with tyrion, and what dany does up till book 5 where she sits on her ass and whines. Far too much of that book boils down to answering the question of what Dany will do.