Using CKII mechanics for Rome would be pretty silly, however, you already, for a large extent, have the hellenistic monarchies covered by the CKII mechanics. I'm not sure how you would do Athens, Sparta or Rome, since there was no central person ruling them: many people were involved in the government of those places.
I am for using characters though. A Julius Caesar type of scenario would not be possible without adding characters, since Rome wasn't at war with those Gallic tribes, but Caesar still conquered them. Another point would be civil wars and rebelling governors. I don't think just spawning a pretender or independence rebel stack would be enough to represent them. Thus we come back to using characters again.
Nobody argues against having characters. Characters are a must. What is being argued is that one must control the state and not a character/family. Just enhance the mechanics of EU:Rome.
As for the Greek states, it all depends on the start date of the game. If it's the same as EU:Rome, then mainland Greece minus Macedon could have some sort of factions like the Victoria series.