It’s that last half-sentence that is most relevant.
Arcane Shot is used because you can’t always use Aimed Shot. When Aimed is on CD, Rapid is on CD, and you have plenty of focus, Arcane is the spammable filler.
The charges are what let movement not actually be harmful in most situations. Since what controls how many Aimed Shot casts we get off is not simply how fast we can cast them, but the recharge interval of the ability, we lose zero DPS as long as we do not permit Aimed Shot to reach charge cap. It doesn’t matter whether you Arcane Arcane Aimed Arcane, Arcane Aimed Arcane Arcane, or Aimed Arcane Arcane Arcane, they’re all going to do the same damage. So if you have to move, and you planned ahead for it a bit, then you simply cast Arcane or Steady while moving, and resume Aimed again once you can plant. As long as you don’t charge cap, there was no sustained DPS loss, because you cast Aimed and Arcane and Steady the exact same number of times you would have had you not moved and done them in a different order.
Also, wow, finally caught up.
Ya, @Dawnspirit, charges are consecutive. If they were independent, you would always gain a charge back exactly twelvesseconds (minus haste) after each individual single cast, regardless of where the other cast fell. If you cast them back to back, they’d recharge back to back at the same interval, separated only by the cast time. If you used them 6s apart, you’d get the charges back 6s apart.
That’s not how it works, and this is readily obvious if you spend some time on a dummy. As Adreaver points out, if you chain cast two back to back, you’d expect to get a charge back 12 seconds after each cast finishes if they were independent. Instead, you get back one charge 12s after your first cast, and you don’t get the second charge until 12 seconds after that, 24 seconds after the first cast. If you cast charges as they come up, never letting them hit cap, you’ll get a charge back precisely every 12s (minus haste) after the first cast finished. When the second cast occurred has no impact at all on this timing or behavior.
But really, the easiest test is simple. Go to a target dummy, remove all of your gear except your weapon (ideally without haste on it, but it’ll be close enough even if it has haste), take out a stop watch, start it, and then cast Aimed twice back to back.
If the charges were independent, each charge should recharge 12s after its associated cast. Since it takes 5.0s to cast two Aimed Shots back to back, this would mean one would recharge at 2.5+12 = 14.5s, and the other would recharge at 5+12=17s. You should have both back in under 18s from when you started the first cast.
If they are concurrent, as we assert, the first charge will come back 12s after the first cast (still at 14.5s), and the second charge will come back 12 seconds after the first charge came back, which will be 2.5+12+12 = 26.5s.
So, should be very easy to see the difference between the two with a stopwatch. Do you get the second charge back after ~17s or after ~27s?
Another test is simply to count how many Aimed Shots you can cast in a given interval (say 60s, or 120s). If each charge were independent, then you should be able to get in 2 casts every 12 seconds, as if you were casting two separate abilities that happen to have the same length cooldown. Each one cools down independently, so you can cast each one every 12 seconds. 60/12 = 5, so you should be able to cast each charge 5 times per minute, or 10 Aimed Shot casts per minute.
If they are concurrent, it should be impossible to get more than 5 casts per minute, except for the first minute where you started with 2 charges. After that, you’d be getting back a single charge every 12 seconds, so 5 times per minutes.
So go to a target dummy, take off all of your gear except your weapon, and cast Aimed Shot every time a charge is available, as soon as it is available, for 120s. Count how many you cast it.
I bet it’s 11 (5 per minute, plus 1 extra charge in the first minute), not 20 (10 per minute).
And if you want to prove it to yourself further, do the same 120s, but instead of casting Aimed Shot as soon as a charge is available, wait to cast it until the button says “6s” remaining on the recharge. Count how many you cast in the 120s interval.
I bet it’s also 11.
If your original assertion was correct (that delaying casting it at all when you have a charge available pushes back further casts), you should see noticeably fewer casts in that second test. You’d be pushing back each cast by 6 seconds, if your assertion were correct, because you’d be letting it sit with 1 charge for 6s (half the recharge) each time it came up.
But that’s not what you’ll see. You’ll see 11 casts in both tests.