I think this was one of the biggest flaws for AC3. While they add to charactaer development, they just take time from you actually playing and enjoying the game. We had instead a good chunk of time playing as Haytham, a good chunk of time making Connor an assassin and THEN we finally had the chance, 6 sequences or so later, to play as an adult Connor finally doing stuff.
I mean seriously, I just wanted to play as Connor and had to do 5 hours of other stuff before getting to it.
You don't truly need those in order though. I mean, Ubi could play it smart, put you as an Assassin directly in game or finishing training or just like Altaïr, who was demoted and had to earn the reputation back. And then play Memories, "Corrupted Sequences" or stuff like in AC:B with Ezio's love interest, instead explaining poarts of the early life of your protagonist.
The good thing about this saga is that the Animus lets you do this stuff in any way you want, really.