I've said elsewhere that I think four levels are needed. As an example, look at four characters' qualifications in Woodsman:
- Myopia: a young alchemy student who's never left her home city and can get lost in a nature strip.
- Grimdark the Blue: A typical adventurer, used to tramping through woods among other environments.
- Sylvia of the Glade: The party's scout, hired because she knew the woods.
- Mick Dundee: justly famed as one of the finest bushmen in the region.
i think every one of those levels is needed in a role-playing game. If you disagree, tell me which one you'd let go.