I don't think that would really work, and it doesn't make that much sense to me.
If you're saying that b/c you're flying SW, if you need to change they make it easy on you so why insure the flights then I agree with you!
But if there's more that you're trying to insure, and there are big cancellation penalities with no help, then it makes sense to insure those portions of the trip. But it doesn't make sense, to me, to insure only some of the people. If it's a package, how would the package be split up? Hotel room is likely going to be charged the same anyway...there are tickets, I suppose, but those can be turned into non-expirations for WDW, can't they? That might be cheaper (I don't know) than insuring them. Rental car? That wouldn't matter if one person didn't make it...
So I say, check the cancellation policies of everything, and if they are harsh and unhelpful enough to make insurance worth it, then get it for everyone. But since SW makes it easy on you if you cancel, I'm not sure if I would ever insure a SW flight...