I thought it was like why Jack had to get his father's shoes -- it has to do with having "something" belonging to or relating to the original 815ers on the plane as much as possible. Someone told Hurley to bring something of Charlie's (or something that reminded him of Charlie --
I think you are absolutely right. Charlie's guitar, Christian's shoes....and like someone before me said---where's that wheelchair?
Don't need the wheel chair, Locke was carried on the plane.
Charlie wasn't jamming out some Drive Shaft tunes on the plane either.....but they still need the guitar. I think the point is that they need to make it as similar to 815---and there WAS a wheelchair on the plane.
The way I see it, the guitar was more of a Hurley paying tribute to Charlie kind of thing as oppossed to actually needing it to be on the plane. And as I pointed out in some other post somewhere, if they are being nuts about recreating it perfectly, then the guitar should have been checked, because Charlie had to check his, but it wasn't, so right there is a mistake in the recreation.
And as far as the wheelchair being on the plane, yes there was one. As Mrs. Alpert pointed out on the last page, Locke's wheelchair was stowed away in the belly of the plane or something, because Locke was carried onto the plane. So, since none of the Oceanic flyers knew that Locke was even in a wheelchair (except for Rose, but she doesn't count because she's not on Ajira 316), they wouldn't have known to make sure that a wheelchair is on board the plane, so as far as they know, they DON'T need a wheelchair.