Depends entirely on the subject. For subjects where the questions that can be asked are limited, the answers are likely to be more specific (eg chemistry, biology or physics, business, economics, IT, etc. You get the idea.) and so knowing exactly what the examiner wants you to write is going to be advantageous.
Where there is much more freedom in what can be asked (eg History, maths (though the technique you should be using can often be seen in the mark schemes), foreign languages, literature, philosophy, ethics, etc) then there is not really anything to be 'learned' from the mark schemes other than the correct format or layout of your answer. For these subjects I think it is better to just practice, practice, practice, and then practice some more. To be confident with maths you need to understand the concepts, and to be confident with essay subjects you need to have the knowledge, of course, but you need to be able to write like a machine; the ideas are not going to get marks if you cant get them down on the paper!