What about: It would take a decade for a normal player to finalize their gear and pushing against the upper limits of human patience / healthy playing habits? Is that motivation enough to increase the rate? (We're all forgetting that you still have to find gear worth Sphereing...)
Some people might be okay with a game in which they can dump continual effort and get almost nothing in return, but I'm a very busy guy, as are many people here, and I like my time spent to feel fulfilling.
If I fire up D3 and I don't find a single good item, at least I gain some Paragon Experience while I play. PSO has nothing like this. Increasing the PD rate would partially fix this emptiness.
If I have all Lv. 200s and I log in and don't find a single useful item or PD, it's basically like I didn't log in at all. And no, I don't need more characters. I don't WANT more characters. I want to play the game at a high level and enjoy the power I've accumulated while still having some way to make progress that isn't so boring and slow that I just log out after 20 minutes. I want to love the game again.