If you look at mormon or LDS students that go away on a two year mission (usually 19-21), they are far more mature and focused when they enter college. Graduation rates are higher and problems are lower. I also feel that at 18 if you get a small taste of the real world, you will value higher education more.
I don't believe that a year gap is very beneficial to a student. I think that the gap just makes it harder for them when they restart school a year later. They will also be at least a year behind all their peers. Too often, it becomes a convenient excuse not to go back at all.
Simply making students wait a whole year before they go to college does not mean that they will grow personally. It is possible that in college that they will grow personally as well in fact they might even do it more. In the year wait they might not do anything at all.
The gap year for students in between high school and college may help a student hangout with friends for a year, but the facts point towards reasoning that suggest that a year off hurts college admission chances because students often find a job and do not return to college. Also, colleges offer a diverse culture unlike any other that helps students grow at a faster rate than any other time in there lives due to an exposure of students of many different backgrounds.