One of the ways I look at it is the more things we get turned into cargo the better. Cargo (or cargo via limpet, like in fuel transfer limpets), is really the only non-violent way we have of interacting with other CMDR's, so we should maximize that as much as we can.
So with the shield cells for example. Say you have an Eagle in a CZ, but you only could spare a C2 slot to feed your shield cell - only four cells (what you would have now anyway with a current SCB, but whatever). What do you do when you run out? Right now, you have to leave the fight, go dock, and rearm.
But if it was cargo, you could have someone standing by in a T-7 (NPC or player even), back away from the fight but still in the CZ, loaded up with spare shield cells. You run out, you just fly over to him, he ejects a few for you to scoop, and it's back in the fight! We not only got you back into the fight quicker, but we legitimately gave a T-7 a role in a Conflict Zone.
And that creates other roles too: That T-7 is a tempting target. Better put a couple Keelbacks (with an SLF each) around it for defense. Maybe those Keelbacks could have mine launchers and set up a defensive perimeter too. Now we're making it interesting. If you like that T-7 feeding you shield cells, you better make sure you take out any enemy ships that get close to it!
And your enemy has something similar too, so if you wanna win the fight you need to take out his supply chain. Maybe you get an in-CZ mission in your Eagle that your side is going to hit the enemy SCB-supplier ship, and it's your job to zip in there and take out the mines so the attack can get though. Or maybe you need to take out the Keelbacks defending it. Maybe they want you to pop in and attach a hatchbreaker limpet to their cargo ship, and so they have nothing left to give their cell-less ships. Or maybe their T-7 isn't here yet, but it's coming; get into supercruise, make sure your interdictor is active, and make sure it never makes it to the CZ! You do the mission, you get given a bunch of free combat bonds for completing it.
I mean, this is getting wildly off topic now, but that's just a couple off-the-top-of-my-head examples of how something as simple as making shield cells a cargo hold item can greatly expand and enhance gameplay, create opportunities for new mission types, new objective-based gameplay, new player interactions, and enable new emergent gameplay.