Well they can't off a kid I don't think that would go over very well
plus Walt has been quite key in many ways. Now that Shannon is Sayid's woman and Boone's sister she has quite the connection built up also she is basically the only other female "sex symbol" on the island with too many single male leads they are going to need her presence unless they start to build up one of the back ground characters. I'm thinking Jin as well he's the only one who can't communitcate with the rest, he is the most isolated from the group and has just broken up with his wife.
It's a shame though because his presence makes this feel far more realistic to me in that not everyone on a plane is likely to all speak the same language, and he presents an interesting challenge to the rest of the group due to his culture and lack of English.
