It would be nice to have a SKU field even when you don't use the connected Product. This allows for more freedom when creating invoices in external systems.
Btw, I do not mean the combo but individual items.
Now you can use product attribute combinations. It works fine (supports SKUs)
It would be to complex to manage if implemented some other way
What you get is one single SKU. The whole point is to keep track of what the SKU.s of the predefined values are. How else are you supposed to keep track of what article to invoice?
Using connected products doesn't work when you have different VAT:s either, otherwise that would be an option (there is actually an old forum post that was never answered).
I see it as a simple case of another column :)