How useful are introductions and conclusions?

This is always a divisive issue as some will tell you they're great and others that they're not necessary, and in the end it tends to depend on where you're using them and what for. Ultimately, I would say this: don't use an introduction to set up the context or waste time with darting around issues but be decisive. Your introduction should be brief. In an exam you don't have time to lose so make sure that you address a couple of key points very quickly but make your judgement absolutely clear from the get go. Address the key words in the question such as "extent" or "how far" and don't dance around your decision. In the same way you must continue the essay in the same vein and don't change your judgement by the end. Likewise, your conclusion ought to be brief and cover key points that you made in your discourse but ultimately reinforcing with incredible clarity your judgement.