Can someone explain to me how the GMS version Light-Dark System works?

Title, but I don't really understand how it works. I understand that I should balance my Light/Dark skills, but what happens if I don't? Also, what happens when I fill all of the feathers of a side of the wing?

When you have at least 1 feather on either side, you can switch to that one.

Using Light skills charges the Light side, using Dark skills charges the Dark side. You also get 50% bonus damage to either side for having it active.

in 3rd job, when you switch, you get a temporary 100% increase in damage on all skills (it's like, 5 seconds, with a 3 minute cooldown).

Since equilibrium's pretty useless, there's no incentive to switch, ever. Just use whichever one has a better skill.