@Sea:
Read my post again. I added many things.
Rayleigh only played role in how the story is played out. Ace and WB set up the frame. Rayleigh was never what the story is about. Ace and Whitebeard were. That's what defined their importance to the plot.
Not necessary to have a character did all what he did. Like a random ship-coater did his job. Jinbei came up with the message. Kuma intervened before Rayleigh came, etc.
That is just how it's played out, not the frame.
Ok I just read your full post and this.
There are 2 big distinctions you are making. The first is the dramatic weight and depth a character adds to the story and how that factors into importance. The other is how central they are to the story and the main conflict, and irrepleaceable. If this is wrong, you can stop right here and correct me.
In the first category it´s true, Rayleigh wasn´t given as much development. So as a character in that respect he is lacking when compared to WB.
Also in the first category you mention that an important character adds depth to the story. In this I have to say that while Rayleigh did not enrich the conflict of any character, or made any emotional sequence mean more due to his presence, he did add a lot of insight to the story, and some nice interactions and reactions from the crew, especially Robin, Usopp and Luffy.
But this isn´t really what I like about Rayleigh. As I touched upon briefly before, Rayleigh is a symbol of the last generation, just as WB is. The fact that he made a similar commitment to Luffy quite out of the blue is something that I found very interesting. This to me made made Rayleigh a more two-dimensional character than his usual old wizard shtick. But so far nothing that makes him comparable to WB.
On the second category, how central to the conflict and irrepleaceable he is though, I disagree mostly with how you view him. You say he is replaceable. Would you have taken his advice and his tutelage on Luffy half as seriously if it was Jimbei instead of the Pirate King's first mate? What would have been Jimbei's motivations toward devoting 2 years of his life to training a youngster with potential, especially when duty called elsewhere? Would Jimbei feel compelled to entrust the problem to the next generation when only someone of Rayleigh's caliber would know what rests in Raftel? Would Jimbei have involved himself so?
I ask you, what other character would have similar motivations to Whitebeard who did not know or experience similar things and had similar experience and power? Why would Jimbei come up with the message, and how would he have known that they were in the ideal place for their growth? If Rayleigh hadn't come along before Kuma relayed him the message, the plan would have fallen apart.
So why does it all tie in correctly? Why is it that Rayleigh, though admittedly not central to any conflict, is just the right choice for all of the things he did? Because it works. Other scenarios wouldn't work as well. He is a necessary character, and Luffy feels more formidable now because he was trained by him, and it is Rayleigh, and his knowledge, previous contacts, and strength that allowed for everything that lead up to his conception of the plan. And the plan to train for 2 years is extremely central to the plot and the change of the Strawhats and their experience.
I believe that Rayleigh is an essential character for the role he was cast upon. That's why I believe he is an extremely important and irrepleaceable character.
Ace I am not arguing against. He was the main character's brother and will continue to drive him forward so his role in death and his importance towards the main character's growth is unfathomable .