If a question has a Balsamiq Mockup within it, rather than having to download the bmml source then edit then re-upload, wouldn't it be better to be able to modify the original poster's Balsamiq Mockup within your answer, sort of like how on forums you can quote other posts and modify it within a response?
Yes. Perhaps when you click the Balsamiq icon when answering a question, the site could give you options to select (one of) the OPs mockup(s) or create one of your own.
As a temporary workaround, you can always just click "edit" on the question, copy and paste the text pointing to the mockup into your post, and click the link to edit the mockup. (When you save your changes, it will get a new ID so that you're not overwriting the original. I've done this when creating multiple similar mockups in my own posts.)