If he was willing to take Charlie's place why even tell Charlie? The wierd thing about his visions is that if he didn't have the vision of Jin, Charlie and Hurley with him 'camping' they would never have gone camping. If he didn't tell Charlie about this latest vision, he would've never volunteered. So his visions aren't really of stuff that is going to happen in the future. It is more like he a vision of a possibilty or something. Things that will only happen if Desmond does this and that. Time travel etc gives me a royal headache, all the paradoxes... ugh.