If you're just starting or still fresh, the time and investment into SR weapons will save you time in itself towards being able to clear harder difficulties and being able to reach the ability to auto battle farm. Early on, if you play with the mindset of only using SSR weapons, you're likely to drag out not finishing weapons and eidolons while forcing yourself to waste more important resources like elixirs and seeds because you're not able to farm efficiently (low PP gain from raid/union events) or not able to farm the highest level difficulty (running expert cause you can't do ultimate in advents).
No ones wants to have to run hundreds of ultimate advents on manual, let alone experts. The amount of time you could save on that by simply investing in a few SR weapons instead of waiting for a new event.
If you're to the point you can clear anything, then it honestly doesn't even matter what you do,