There's a very worn out question that everyone has asked or been asked at least once in a lifetime:
"If you were stranded on an island, who would you want there with you?" Or something to that effect.
So take into account that our little ship may not make it to Aman, since it is layered down with a multitude of characters. Then you would want the unpleasant characters (i.e. those who would: eat/torture the sanity out of/steal the soul of/kill you) to be pushed off the ship early in the journey, right?
Wrong. They are the ones you want on the theoretical island with you, because the tenacity of the "evil" characters would undoubtedly prevail on a deserted island, leading to a devious master plan to get off the island and on to Aman where the real fun can start! All you have to do is stay alive, sane, and hang on to your soul.
If you were with the "good" guys, yes, you would escape the island, but it would also take far too long and put you through much angst and pain.
So, in answer to the original question: all the good guys, even those whom we're not sure about, like Boromir.
Erestor gets to stay on because he has nice logic. A bit like mine...
