Ailerons????
i beg to differ with a lot of the comments on this subject.
A rudder and ailerons offer different control aspect in flight.
A true glider should have a full house of controls.
If a simple glider has enough dihedral, than it can achieve some of the banking as a full house glider does, but it cannot respond nearly as fast as one with ailerons.
Rudder is yaw...keeps the nose pointed into the relative wind. Combine with dihedral and elevator, the glider can bank.
Ailerons control roll. In order to turn an airplane properly, especially one without excessive dihedral, ailerons, rudder and elevator are needed. Exception, canard airplanes.
To suggest that most gliders only need rudder and elevator is completely incorrect, and an insult to what gliders are capable of.
In full siize sailplanes and in complex models, banking angles in excess of 60 degrees are common. The tighter the thermal, the smaller the core is and the more bank angle is required. This is also true when closer to the ground or the source of the lift.
When banked over in excess of 60 degrees and the thermal kicks your tail around, or you leave the lift and need to get back into it, you need as much control as possible to keep control and get back in or remain in the lift.
Here is a glider with a lot of dihedral and a full house of controls. The second picture offers a view of a relatively tight turn. The yaw string on top of the canopy allows the pilot to keep the nose pointed into the relative wind and that is controlled by using the rudder.