On my course my university provides past exam papers but does not release mark schemes and I know this is the case for other universities. I am dyslexic which makes reading hard, and for my high-school exams I used exam papers with the mark scheme as a self-study method, (meaning I attempted questions and used the answers as a source of information on that topic). Which leads to what I see as one of the main arguments against not providing mark schemes; by providing exam papers you give students an idea of what they need to study but not the information they need to know (i.e. the answers) meaning they will spend time (which could be spent on learning other things) looking in text books for answers.
So what are the reasons for not providing mark schemes to past exam papers?
Just to clarify some points:
Firstly I am from the UK hence the phrase 'mark scheme'. When a wrote the question (which was a while back know) I guess I was more tending towards the idea of a mark scheme as example solutions rather then a 'grading rubric'. I must admit 'mark scheme' was probably the wrong phrase to use in the question. Just to keep the question consistent with the answers, the interpretation of a 'mark scheme' as "any method that allows the student to work out what they are expected to write in the exam" works best.
Also a while back I did email the relevant people at my university about the situation concerning 'mark schemes', also explaining about my dyslexia and how I learn best. They replied by simply saying that the review the situation every year, but gave me no further information for the reasons behind it.