Here is my issue: on a website I maintain, the owner has some 150 or so albums on Google+. Although all are technically public, she often doesn't want all of them to be on her WP blog — but just some of them. Say, just 120 of them :)
It's always the same 20 or 30 that are left out, and newer albums (she is adding them all the time!) usually are acceptable.
What this means is that the shortcodes for achieving this have to painfully include all the 120 out of the 150 albums she has. Every time she adds a new album, all shortcodes have to be manually updated — on several spots — which is painful.
It would be much easier if we could have negative ids to exclude ids. That way, every time a new album would be created, all that would be needed was to refresh Shashin (and eventually clear caches). Only when there was a new album to be excluded would the shortcodes need to be changed, but in this case, this is a rare occasion (while adding new ones happens every week).
I understand that at some point the code will call a WP_Query somewhere. WP queries usually allow negative indices to allow exclusion. So this shouldn't be very hard to do for the shortcodes as well.