You can apply psych through every aspect of nursing, not so with some of the more specialized areas. You may not like in-pt psych, but trust me, what you learn will come in handy down the road.
The purpose is to give you a good foundation. After that, you can choose to go where you want. If schools allowed students to specialize while in school, students wouldn't get the general knowledge they needed. No matter what specialty you go into, you can always carry basic med/surg knowledge with you and apply it. If you specialized right away in say, OB, you'd be out of luck if you ever wanted to do anything else, or if you graduated and found there were no openings in OB areas.