To what extent is knowledge (true belief) instrumentally valuable? Is it always instrumentally valuable or is this not the case?
Knowledge has instrumental value when it serves a purpose, in philosophical terms this would be expressed in terms of 'teleos'.
I will first look at the question from a practical standpoint and then consider some other arguments for and against the intrinsic value of knowledge.
On a practical level, there may be cases where having knowledge can be counter-productive. Prichard gives the example of someone needing to jump across a ravine to get to safety. If they have the knowledge that they are unlikely to make it then they are less likely to succeed if they hesitate as a result of having this knowledge. However, I think this example is an exception which needs further examination. In almost all cases it would be better to know if you were able to jump the ravine. Furthermore, if the knowledge of your slim chance of success was also accompanied by a knowledge of the psychological effect that this knowledge causes, then it is possible that the person could overcome their nerves. This is to argue that knowledge is not harmful if it is accompanied by sufficient additional knowledge. Furthermore, if they really had complete insight they would never have got themselves into the dangerous situation in the first place!
The phrase 'Ignorance is bliss' expresses the thought that it is sometimes better not to know. If one takes a utilitarian view of life in terms of the Greatest Happiness Principle, then it is apparent that some knowledge is better to be left undiscovered. Both on a personal level and on a societal level we can all think of examples of knowledge which detracts from the sum of total happiness. It might be thought that mankind would be better off in the long run if it never acquired the ability to destroy the world - for example the knowledge required to manufacture nuclear warheads and to convert fossil fuels into energy on a massive scale.
I also think that it can be argued on a deeper level that not all knowledge is intrinsically valuable. Learning for learning's sake expresses the view that there is some value in acquiring knowledge for its own sake. It could however be the process of acquiring knowledge that is useful and not the knowledge itself.
It seems to be human nature to want to acquire knowledge, whether this is driven by 'nature' or 'nurture' it seems to be part of the human condition that we value knowledge over ignorance. There are however cases where knowledge is not valued. It could be argued that many religious beliefs are very highly valued but are the antithesis of knowledge, since they cannot on the whole be classified as justified true beliefs.
In order to adequately answer the question of whether all knowledge has instrumental value I believe it is first necessary to agree what is the purpose of life. This is of course an almost impossible question. My own view is that life does not have a purpose, over and above any purpose we ascribe to it. Richard Dawkins expresses this view very elegantly. He asks why we suppose that life should have a purpose any more than why a mountain or the colour blue has purpose.