Well I think we should go back to the real topic.
Well, starwars is written in a format not unlike ancient mythology. And coincidence is the key ingredient of a myth.
Drama, Karma, Fate.
To get this working you NEED those coincidence in the main story line.
As for the rest of the stuff? well, cameo are just cameo, which is always good w/o ruining the story. As for the rest, I do think some of them are done "for the fans" as some crowd-pleasing act (think "this party is over").
Now, RL coincidence do happen in many history changing events though, such as the Lincon/Kennedy thing, and I don't think Lucas did that. Maybe its just that people would try to dig out coincidence in a big event, such is the focus of a saga?