Yeah it does open up at that point but to be frank the side-quests are so fucking hard there's no point exploring Grand Pulse until you've finished the main story, in fact you can't even reach your maximum role levels until after you've finished the story because the last levels of the Crystarium leveling system are locked - this is the sort of stuff that annoyed me about XIII, the upgrade system and shops were also pretty useless throughout the entire story mode, I finished the game without visiting the shop even once.