They could if they wanted to. It's present in FFXIII-2 (controllable story outcome), however in terms of voice acting, all MMOs that have some amount of it for even generic quests reuse voice samples when able (especially the bigger the game gets), so it wouldn't add too much when you get to (new town on other side of the world) and quest NPC sounds like the dude you helped in your first 20 minutes of playing.