I for one would also love to have the ability to use the slab versions of the cargo block, I wonder if there is a way to allow the use of the slab slider with cargo modules so you can choose to make the slabs the same way you would any other slab. That would not be confusing to the player as it's consistent with other slab mechanics.Technically that would classify as a bug. A while ago we made sure that blocks that show no difference in orientation (like hull) are also not rotatable.
The rotation of a cargo block is what determines the slab you'll see. Which may be useful for decorative purposes but for people that don't know you can rotate it (since there are no arrow keys, only way to do it is through the mouse wheel), it would be confusing/frustrating to them so in this case I would prefer they can't be rotated at all, it would always show a full block.
I'm not sure if we want to make a separate non decorative block either...The only argument I still have against non linked full cargo blocks is that it confuses people. Having a separate, purely decorative block would probably even worse. As in, you can make "fake" storage areas which will make shooting cargo on purpose impossbile, you never know what you're hitting is real cargo or just decorative. For now that's not a big deal, but perhaps it will be later.
A possible future situation: raiding a big trade ship and you can't overheat it in time before it jumps away, you might still be able to hit their used cargo blocks (either by accident or on purpose) which would cause them to drop tons of items. You don't get to destroy the ship, but you still get some great loot.
Considering you can have empty boxes in real life I don't see how allowing empty cargo containers would be all that confusing to players as long as the method to place them is consistent. No connection = full cargo block (slabs if at all possible). Linked cargo = filled according to cargo computer.
Yes shooting a ships cargo as it jumps away to gain loot sounds great but un-linked visible cargo doesn't really take away from that. If cargo does actually dump items to space when destroyed in the future it also opens up counter play. Have some valuable cargo on your ship or station? Hide it among a ton of empty boxes. Want to ambush unsuspecting would be raiders? Put a ton of extra cargo to make your ship look full and therefore heavy and surprise them with maneuverability.