I'm trying to understand why you think this is a good feature put in "by design."
Consider:
I add 20 images to a directory, batch build, then add descriptions to each.
Oops, I need to add another image I forgot about. No problem, I'll just add it and do another batch build.
Wait! I can't do that or all my work of adding descriptions will be lost!
So either I can't add images to the group, or I have to redo all the descriptions.
This is a feature "by design?" Sounds like an oversight to me, and something that needs to be changed.
What am I missing?