You're not exactly going to be "chaining" the Ultimate Weapon quest for your free Fantasia if it resets everything, every time. How many times do you want to go through the ARR MSQ just for that - all those long annoying fetchquests that everyone hates - and still need to get back through Heavensward and Stormblood before getting back to the current point of the MSQ?
If anyone wants to put themselves through that repeatedly for a single Fantasia each time, I think they'd be welcome to it.
Although if you
just want to go over field dialogue, the quest scripts on
Garland Tools are fantastic for that. Not quite as nice as seeing it in context, but a lot easier to revisit!