Having acknowledged the limitations of the historian's knowledge, though, the question is what do you do next? A good historian will use the fact that he can't divorce himself from his own viewpoint as a reminder to be extremely cautious. A bad one will use it as an excuse to indulge his particular biases.

One man's freedom fighter is another man's terrorist. Criminals often think they're justified in the actions they take; in fact, often they must self-justify their own actions in order to sleep at night at all, so they will have their own internally consistent narratives. It's extremely difficult to fight the dominant narrative once it has become accepted.

IMO, the highest value history has is in telling stories about a world which is different from our own, but for which we have somewhat believable evidence actually existed. The degrees to which it is different tell us what varies in human nature, and what stays the same, stays the same. It teaches us to not take the present moment too seriously as some kind of apex or nadir. But I don't think it actually tells us a whole lot about the past, per se.

And a very bad one will pretend that he is being value-neutral and objective, and show no attentiveness to the interpretative nature of his work.

I'd say this is also true in the broader sense of how a person chooses to live in an inherently muddled world. You can take obstacles and confusion as a call to action and vigilant reflection, or you can use them as an excuse for giving up and accepting mediocrity.