I doubt that Kishi just threw that fact in there for the sake of character development. They don't have to play an important role, but do you think that they will be mentioned again later on?
That was to show that even among Uchiha, good people are born....
But Itachi was an exception because he was born in the house of the most lowly Uchiha of that time(Clan leader staging the coup)
well if you look at the history, every time shisui has been brought up, some kind of history has been revealed, so unless theres anything else, which possibly madara talking to sasuke could bring him up, i dont think theres any more need for them.
No they aren't important to what's going on...Tobirama was just making a comparison between itachi and kagami uchiha because they shared similar ideas and beliefs towards the village of konoha...since Tobirama never met itachi he just mentioned someone like itachi who he knew very well