Chucking money at them is probably the best way to ensure new expansions, and therefore character storylines. So if you want that to continue, I'd buy it. I get the idea that Chapter 4 is in the works as developers have been talking about new story, but I doubt it will be produced if BioWare simply aren't making enough money.
I love this game. I'm not buying this "expansion".
I play this game for the character storylines. They aren't continuing so, sadly, I will most likely stop playing.