Mountain bikers are passionate about the gear they choose to ride, and will take to the grave certain choices. Pedals are among those preferences, and many cling to a clipless pedal system with ferocity. While the reasons might seem unclear at first, once you discover your preferential pedal, you might have similar feelings.
For the budget-conscious rider looking for a great first experience with clipless, the Shimano PD-MT50 is a great start.