Even if it does look silly, when was the last time you needed a full belt's throughput of batteries or potions? Those are very valuable items and stacking them will get very expensive very quickly. It's only natural that players would find no value or need in stacking them. It's one of the reasons I think normal inserters should not have stacking output, partially because there are so many places you will never need stacking power and partially to make sure the stack inserter stays worth the expense.Yes, but with iron plates, an object that looks mostly flat. I bet it wouldn't look so neat and clear with items that aren't square and flat, like ores, batteries or science flasks.
(I have no strong opinion on allowing inserters to grab stack INPUTS. I think it's safer if they can grab items since the real difficulty is stacking items on the belt in the first place. Also if the player drops items as a stack you don't want that to immediately break your base.)
There's probably a UPS benefit to separating the options as well. A normal inserter only has to look for a viable horizontal space on the belt to place an item. That's one CPU test. An item stacking inserter has to find a horizontal space, or it can find a matching item that can go taller. That's 3 CPU tests. I don't want to speculate on how much CPU time it costs to play with a stack, but for a hyperbase every little bit counts I guess? I don't know much more than that.
With an item stacking belt system the item stacking tech would be unlocked somewhere with stack inserter research. For speculative numbers the stack inserter would peak at 12 item capacity and then 4 or 6 belt stacking capacity (3.0 or 2.0 belt unloads; upgraded inserters drop 3 single items). Super stack Loader research would be FAR beyond that, somewhere near end game tech or post game tech since you don't really need it any earlier than that.