I think I may have stumbled upon a glitch that gives you virtually unlimited ATB bar, though I'm not totally sure. 
 
It SEEMS like whenever I switch paradigms just as my controlled party member is doing their last attack in the chain, when the paradigm animation stops your ATB bar is full, allowing you to bust off a full bars worth of attacks instantly. And then of course, you can repeat at the end of that combo, and just keep attacking non-stop. Is this a glitch, a feature, or am I just imagining it? I can do it most consistently when I'm lighting when she has a 4-bar ATB guage. If I time it so that the paradigm switches when she's in mid backflip, it works every time- the enemy even takes damage from the 4th attack. This works best when you use the 'repeat' function- just make her repeat 4 attacks, so you don't waste time selecting them. Also I should note, it only works from the second paradigm switch onwards- when they use the shorter animation.
 
Anyway, I thought i'd tell y'all to see if there was some reasonable explanation for it. Otherwise, try it out for yourself. 
 
I play the xbox version if that changes anything.

I thought something like that might have happened to me during some of my battles, though I wasn't sure if it was part of the gameplay, a glitch, or if it just was filling up normally by the time my fourth animation finished and I had selected my next set of attacks.

I'm pretty sure it's not just it naturally filling up, because if you get the timing wrong the bar barely fills 2 slots when you return back to the gameplay.

It was an intentional battle mechanic. Fans dubbed it ATB-Cancel.

I have noticed it isn't every time you switch Paradigm's but it seems to happen after you shift, wait a period of time (about the time of 3 ATB bars), and then Shift again it will be full.  I also noticed as you did that it seems to coincide with the short shifting animation (everyone changes at same time rather than 1 at a time).  It can definitely help in some situations but sometimes it isn't worth the constant switching. 
 
Also, if you change right at the start of battle it will fill your bars.  I like to switch to a Paradigm with Sazh as Synergist right at the start of battle so he can start hasting everyone asap.

It just naturally fills up, the paradigm shift doesn't pause the combat, you still get hit during the switch. so you're our ot control for a couple of seconds.

" It just naturally fills up, the paradigm shift doesn't pause the combat, you still get hit during the switch. so you're our ot control for a couple of seconds. "