I generally sync music once a week but sometimes I just want to add a few songs/artists/albums. The only way I can see to do this is by unchecking "autosync on connect" and "delete files not in the sync list" and unchecking all the smartlists that are normally synched, otherwise MC does the whole sync from the beginning which takes at least 20 minutes. And when it's done I have to go back and recheck all the usual options.
An option to sync only selected files and leave everything else alone would be very nice.
BTW I tried a workaround by creating a smartlist that contains all the playlists I usually sync, which would allow me to only uncheck that one box when add just a few files, but when I use that smartlist the playlists are not recognized on my Samsung. The files get synched but I can't see them as individual playlists.