Frank memorized the manifest so that when he arrived on the Island, he would know if he met a survivor of Flight 815.
Frank memorized the manifest because he was guilty about Seth Norris crashing the plane where-as Frank feels that he could have made it to L.A.
Frank is On Charles Widmore's and Eloise Hawking's "Team"
Frank says to Sun "they say they're the good guys... I know, I'm not buying it either." This line is significant. Either, the producers are telling us that we shouldn't assume that Ilana and Bram (and incidentally, Jacob) are "the good guys" just because they say they are, or, Frank is undercover and actually on the side of Charles Widmore/Eloise Hawking.
This could be the case because (1) he flew Widmore's helicopter to drop the freighter crew onto the island and (2) he was somehow substituted as the pilot on the Ajira flight to fly the Oceanic 6 back to the Island. Oddly, he always seems to be the guy who flies people into the island- an island that shouln't be that easy to fly into.
Widmore and Hawking aren't necessarily the same "Team"
Frank was arranged to be on Flight 316 by Widmore, possibly via Sun. This would give rise to the idea that Frank and Sun have left together on one of the three outriggers on the beach near the Hydra Station where they landed. These two are likely plotting against Ben on behalf of Widmore.
Frank said he makes the run to Guam all the time.
The reason Frank didn't fly is because he overslept that day.
He is a candidate to become a member of the group Ilana/Bram belong to.
Ilana knows that John is dead, so she thinks Frank could be a Candidate for the new Leadership role of the others. Frank was supposed to fly 815, but he didnt. When the monster found out the pilot wasn't Frank, it became angry and killed Seth Norris, because it knew that someone else would become the new leader of the others instead of Frank. That someone is Locke.
Just a pilot but essential for the escape of the Man in Black
Frank Lapidus is the only one who might be able to start the Ajira plane and take off the island, after the plane has been turned around.
He is thus essential for the physical escape of the Man in Black. Ilana is fully aware of that, which is the reason why she makes sure that Frank stays with her group so the Man in Black won't be able to get off the island.
If that were true the Man in Black shows an alarming, irresponsible lack of interest in Frank Lapidus
Or Frank is not the one necesary to later help the Man in Black get off the island but Ilana and her companions
All things considered, that plane is not a viable means of transportation. If it was, Widmore's group would have dismantled it.
The Last Recruit
Flock's goal is to fly everyone off the plane. This will require a pilot. Lapidus is the Last Recruit (the last person needed by Flocke). The episode on 4/20 will be Lapidus-centric.
Frank is not the only remaining survivor who knows how to fly a plane. At some point Jack 'took a few flying lessons' but it 'wasn't for' him. (Pilot, Part 1) He may know enough to fly the plane off the island if it came down to it.
A "few flying lessons" hardly qualifies Jack as being able to fly a commercial airliner off a hastily-built run-way. We know that it wasn't long enough to accommodate the planes landing.
The last recruit is Frank but in reference to being Jacob's last recruit. Frank is going to have been talked to by Jacob, but has never been talked to by The man in black. This is essential to his ability to kill him, just as it was with Jacob. Ben was able to kill Jacob because he never let him speak to him, and every other character has been spoken to (and decided to go talk to) the man in black. They are, as Claire put it, "with him" - except Frank.
Ben DID let Jacob speak to him beneath the statue: Jacob tells Ben that he has a choice on whether or not to kill him.
Reason for not being in the flash-sideway
Lapidus is immortal. He survived a plane crash, two helicopter crashes and a submarine blowing up.
Lapidus simply just moved on. He had no need for remembering.
Frank moved on in the Flash Sideways with a different group of people. Perhaps people of greater importance after he flew the surviving Losties off the island. I'm sure they must have delt with a lot of problems after leaving. A big plane with only 6 people, who mostly weren't originally even on the plane, must have caused a lot of suspicion in the off island world. Frank must have gone through a whole lot more before passing on.
Frank had no regrets when he died, so he did not have anything to let go.
Frank's purpose is to bring people to the island
1) He piloted the helicopter to bring Faraday, Miles, etc. to the island. 2) He piloted Ajira 316 to bring Ilana, Jack, etc to the island. 3) He was supposed to pilot Oceanic 815 to the island. This is why the Smoke Monster/MiB killed Seth Norris, because he was never supposed to be on the island, it was always supposed to be Frank.