Wyatt is on the same topic I'm on about this week, it seems. Typing some very similar noise yesterday.
Though I imagine for sandboxy games, saying "My Campaign is About X" isn't the most useful thing to spend time doing (and really, any or all of the plots that he enumerates can be present in a sandboxy game -- in fact, the more the marrier!), for more narrative games, it really helps define the central conflict.
What I find is really useful is then to tie this back into
character creation. If your campaign is about the conflict between the gods of Order and the primordial forces of Chaos, then this should echo in the characters that are played in these games. Backgrounds, classes, feats, whatever, any character made in such a campaign should bear the mark of that campaign. If you make Joe the Fighter in that campaign, then Joe should have some distinctive element that ties him into this conflict between divine order and elemental chaos.
Where that gets interesting is when you have an open field, and let the players select the conflict by choosing character elements. If you wind up with a party that selects elements related to evil deities, elements related to wilderness survival, and the terror of mortality, you have a ready-made campaign theme: evil gods of death and nature!
Which ties into the emergent quality of themes. Sometimes, the theme of a campaign isn't set from the outset, but emerges over time. This is perhaps another benefit of the first three levels in NEXT: lets the themes emerge organically in play.
I like themes -- conflicts -- as a guiding force for narrative games, and I think the ability for sandbox games to a la carte their themes is pretty strong point in favor of their inclusion, too.
Personally, I've done a lot more with what he calls "literary" themes than with just big epic central adventures, but I've also done games where I just make things up as I go along. I'd appreciate advice for literary themes, conflicts, etc., and as long as it's a light touch (with maybe some modules), I'm in.