Should previous final exams be released to students? In general, you can find the past exams for the past 15 years or so at my school. However I had one prof in Calculus III who chose to withhold his exams. This was the only instance in which a prof has chosen to do so in my two years at university studying engineering. His justification was that he didn't want students studying exams rather than the actual course material which I find to be interesting. 
As a student, I notice that solving past exams from the same instructor as practice can be helpful for a couple of reasons:


it helps you get accustomed to the instructor's style of exam so you're less nervous when you're writing the actual exam. I find that even seeing the familiar cover page helps in calming me down just before an exam starts.
it  helps you figure out what type of questions the instructor likes to put the focus on (i.e instructor A prefers to put conceptual questions, instructor B hasn't put a question on the laplace transform in 5 years so maybe I'll study that later if I have time, instructor C always has a 2nd-order circuit as question 1, etc...)


Of course, the 2nd point is exacly why my calculus instructor chose to withhold his exams. But at the same time, some students may benefit if they use them responsibly as challenging practice problems without jeopardizing their learning experience (i.e someone with the mentality of "I know instructor B hasn't put a question on the laplace transform in 5 years, but I need to study it anyway because it's important"). Most of my other instructors release their past final exams, but do not post full solutions (or full solutions to just last year's exam). A few have chosen to release just the final answers to the questions without the full solution.

Is it better for the students if the instructors released the past final exams? What about full solutions (or just final answers) to the finals exams? Are there studies that show if one strategy is better than another?

I understand that the answers and arguments may vary depending on the field so I should add that I am particularly interested in an answer for the fields of mathematics, science, and engineering.
I don't have an opinion on releasing solutions, but the exams should definitely be released.  Some students will have access to them through previous students, one way or another. Making them officially available makes sure everyone has the same access.