Why is it any more OK to require the world's leading martial artist to be an adventurer?
IMO the solution to this problem is simply that (at least non-combatant) NPCs level up differently than PCs.
Given that class levels are primarily a way to measure combat ability, how else would you make the worlds leading martial artist besides class levels?
Unless you mean he might be a 20th level monk but have never left the Monestary or thrown a blow in anger, to which I reply: Sure. Who said you couldn't? I mean really, do apprentice bakers have to grind rats in the pantry to level up to journeyman? No.
You know, several good points have been raised here. I think the one that bugs me the most (and to which good answers have been suggested) is that the skill system WotC is discussing seems to draw no line between skills which can be reasonably attempted by an amateur, and those which simply cannot be done without the appropriate knowledge base.
Most in-combat skills for example are something that, while training helps, are not impossible to accomplish without it. Tumble, sneak, spot, disarm trap: These are are skills which could be successfully performed untrained with talent or luck.
No amount of natural talent however will let you correctly guess how to speak Ancient High Illithid however, or design a nuclear power plant.
If they are using a 3 tier system with bonuses of +3/5/8 (which I can only pray will be Apprentice, Journeyman, Master and not something genre inappropriate) I think it should actually be a 5 tier system with two different +0 tiers.
- Ignorant: Ignorance of a skill means you cannot make the attempt (or take a -10 or worse penalty) The only word of Orc you know is a mortal insult that you think means please. At blacksmithy you might successfully hit your thumb with a hammer.
- Familiar: Being familiar with a skill means that while you have no special training in it, but it is part of your background and you can make your check without a penalty. For a language you might know a dozen words or speak pidgin. As a blacksmith you could assist with the bellows or maybe straighten a nail or sharpen a blade.
- Apprentice: You have a solid basic training in the task, +3 to attempts. You can speak with an accent and occasional errors. You can forge acceptable kinves and horseshoes.
- Journeyman: You are on the way to profesional level in the skill, +5 to attempts. You can speak flawlessly and with little accent. You made your own blacksmithing tools.
- Master: You are an acknowledged master of your craft, + 8 to checks. Your speech can impress diplomats. You can create mastercraft weapons.
The fun part is that then you can jigger around the 0 state of a skill to express cultural or character background.
For example maybe the Nomads of the Goditzhot desert have never seem a body of water bigger than a bucket but do have trade with the mysterious Djinn. So for someone with the Goditzhot background Swimming defaults to Ignorant rather than Familiar but they get to be Familiar with the Djinn Language in exchange.
It can work for classes too, so probably all fighters should be at least familiar with blacksmithy so they can do their own basic maintenence.