If I export my BSS design, and then I delete some files (in BSS), and then I export my design again: my export-destination folder still has the old (now-deleted) files.
Maybe an option can be added to the Export options to delete the contents of the destination folder?
The only way I could even "remotely" see this as anything that should be added would be if a very large, with large print, warning showed up when it was turned on so that people that have no clue what something does and clicked it anyways, wouldn't lose their entire project of items without realizing it. Trash could be emptied on their computers before they were even aware they had deleted all their items in a folder. Would have to have very good safety precautions in place.
Since a lot of things in this app cater to the "unaware", I'm going to venture that this isn't a good idea. But should they do so, I would hope that good safety measures were put in place for those that won't have any idea what they are doing.