Should I be concerned about student access to a test bank?
As long as the test bank has enough questions to cover the material I'd not be concerned.  Those who work at it will learn.  I've given my own students "sample exams" for years, then put some of the sample questions on the real exam.  The students who were going to earn grades of A anyway got them right.  The students who were going to earn unsatisfactory grades anyway did not.  

A colleague performed the experiment of handing out the course final exam a week before the exam date.  Students were encouraged to get help studying, but then had to take the exam again in class.  He says it did not change the grade distribution at all.