You’re asking LR to delete everything off of a card that doesn’t match something in its library. This is not safe in the simple and general case, because LR cannot know or reasonably verify that nothing has been changed on the card.
For example, if LR is just doing a quick filename match between the LR Library and memory card:
What happens if you put in the wrong card, one that has not been imported yet—everything (on the card) would be deleted.
What happens if you’ve take more pictures and reinserted the card, then those new pictures would be deleted.
What happens if you renamed the photos as you imported them so they can’t be matched by name, anymore—everything is deleted.
LR could detect a wrong card or added images or the renamed photos by keeping track of the card file names and locations, a file-signature computed from the data, and a memory-card serial number for each imported file and check it is only deleting a previously imported file that no longer exists in the LR library. LR would need to retain this information for each original file, forever, or at least an indefinite period of time, so probably would need a way to purge it. This would also help out the people with the opposite problem, where they want to skip any previously imported files, even if they’ve been deleted in LR, because they keep all their files on the card just re-importing only new images each time.
Alternatively, LR could do things safely if the memory card remained in the reader or the camera remained connected from the beginning of the import process, through the review process, and up through the last back-synchronized delete. As soon as the memory card is removed then things are unsafe because LR can’t know or trust that nothing has changed on the memory card between the import and review process, then LR would have to prohibit the synchronized-delete operation.
The point is that LR doesn’t know enough, yet, to keep things safe, or would need to require an unlikely scenario to keep things safe. Adobe has yet to deem such a request worth the extra programming effort to accomplish it safely. It's easy to ask that everyone pay a little extra for Adobe to program things safely for a few people and add extra support staff to accommodate the rash of calls every time the delete-synchronization is no longer available because the memory card was removed or camera disconnected before the Import/Review process was completed.
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“Packing light” seems mostly just an excuse to not change an existing routine out of sheer stubbornness, because an external USB-powered 1TB drive is almost as small as a smart-phone, a pack of cigarettes, a deck of cards, and probably smaller than an extra pair of socks or underwear, so easily can be packed with a laptop.
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