The advantage is (and I should mention that even if you switch synching off in the UPlay Launcher it
still gives you the "please wait while synchronizing..." song and dance when you start the game for reasons unknown to man
) that synching automagically "resurrects" your save games if, for some reason, you've managed to murder or mess up the save directory on your hard drive. The "disadvantage" is that, in order to get old save games to
stay gone if you have to, say, reinstall, is to delete them from the save/load menu in the game and then let the game sync when you start it up the next time.
As the "disadvantage" of keeping the sync on isn't all that much of one, you might as well keep it on.