I like a lot of the ideas and comments offered thus far.
That's what I can think up for now. What do you do with your guide type NPC's? Do you use them?
I like the idea Huss, but I think it depends very much on the function of the guide as to how the guide operates.
A Sage is one kinda guide, who can provide information, research, perhaps other contacts or even real Intel. An Indian Scout is an altogether other kind of guide, who provides in the field, hands-on experience of say a locale, or of native customs, an access point or approach vector, terrain and conditional information, etc.
There is a great difference between theoretical and academic experience (though that is not to say that a Sage is limited to mere academia, he might be a retried adventurer, let's say, who has spent his later years in research and study) in how effective a guide is, and how they operate. (Or how anyone operates for that matter.)
I'd say it also depends (operationally speaking) on other factors such as social class, profession, status, culture, race, personal ambition and expectations, etc.
I do agree that such a guide (and I have used them often, and many different kinds of them, from those who sort of idolized someone in the party, to deceptive guides planted purposely to spread disinformation, to spies, to natives hoping for reward, to party member relatives, to professional guides) can be invaluable to an adventure or a campaign depending on how they are used and what relationship they establish with the party or party members.
The guide should be less competent than the PC's. If the guide is more competent, he stops being a guide and starts being a leader and that sucks when the NPC is the leader. Choo choo! Everyone on the Plot Train. A guide who is weaker, less powerful, and possibly subservient in some way to the PC's makes a much better guide. He can offer ideas, he can offer guidance, which is what you want, but, he can't actually really affect any outcome.
I'd disagree with this a bit though. I've never employed a guide who the players could assume was level so and so (superior or inferior to themselves, in my setting things like class rank or level are meaningless for character and player interactions, you can't look at someone and say they are a 15th level Marine, but like in real life you could tell they were a Marine through manner, bearing, dress, habits, training, etc. - so power relationships are not level dependent) and so never established a working or authority relationship based upon those factors. To me it's perfectly fine to employ anyone of any level as a guide, using your basic definitions, because "taking over" would probably not be a major consideration of any guide. Either they'd not be interested, it wouldn't be profitable, or the idea of assuming control over the party would be non-sequitor, ineffective, or uninteresting, and therefore not part of their agenda.
(Manipulation of the party might very well be part of the agenda and I have used "guides" in this way before, even planted guides from counter-parties. You can manipulate information dispersed in such a way as accuracy and reliability is achieved almost perfectly until such time as you actually want to corrupt or misinform those you are manipulating. That's just basic espionage technique. But clever manipulation is different from direct control through the effects may sometimes be similar.)
I am using the analogy to character-level to basically discuss the idea you presented of competency since so much of the game relies upon level to associate with capability and competency. (That's a gaming issue, but it equates in many settings to a real factor in how many NPCs interact with characters, though not in mine. By that I mean it's just sorta assumed in game that competency = level to some degree or another, that really component people will be of higher level than others, though that's hardly true in the real world. In game a zero level guy is not intuitively or instantly considered "component" but in his field he might be the most competent guy in the nation. This is another way of saying the best Scout is not necessarily the best Captain, but in game terms it is hard for many to imagine the first level guy as being as competent as the adventurer who is level 12. But believe me it does happen that sometimes the recruit is more competent than his Captain. He may just not have the rank or the age or even the drive of the Captain.) But as to the general idea of competency I think that just also depends on what the guide wants and what his real objectives are, which may have nothing to do with direct control or influence over the party at all. He may be very competent in one way or another or may even be the most competent guy around without any interest or motivation of any kind for leadership of, or outstripping the party members.
I'm not sure though that he shouldn't affect outcome, at least sometimes. I think I know what you're saying, but I'm using outcome in a very wide sense, and I'm not sure you are.
Though I think if a guide (or any NPC) from outside were to try and "take control" of the party, or to split it, or redirect party objectives, or assume a position of undue authority or influence, then that would make an interesting sub-plot all by itself. How would the party react either cohesively and as a team, or in a splintered or confused fashion to such pressures? It would be an interesting situation for them to deal with and for them to decide how to address. The role play implications would be strong.
Anyway I like your idea, or rather your basic formulation or formal statement of the idea. this is an old technique I'm sure many have used in many different ways but I think that by addressing it openly you give some people new ideas.
What shouldn't a DM do with them?
I don't think it's wise to use them too often, to solve really difficult questions for the PCs, or to divulge too much information, especially not too easily. I wouldn't use them to "give information" to the PCs if the players would do better to obtain vital information in a different or more challenging way. I like guides as guides, but not necessarily straight-answer men, calculators, or problem-solvers.