I always liked the cameo of Picard in Emissary. Not only does it proverbially pass the torch as previously mentioned, but it serves two other purposes.
One is to give a bit of insight into Picard, you get to see how people outside his circle of acquaintances and people he's comfortable around react to his Borg past and that haunted look on Picard's face. Between Avery Brooks angry glare and Stewart's face deflating so much is said in that one snip without words.
Two is to establish that hey, Sisko isn't like Picard and this isn't the hunky dory TNG crew that sings kumbaya in the meeting room. Sisko's had a rough past, with a very physical manifestation of what he lost standing right there before him. He hasn't led the comfortable life, is a single dad, and between getting exiled to a junkyard station and having your wife's murderer be the one that's making your child grow up in that cesspool is it any wonder Sisko wasn't happy or less than civil?