In small airplanes and especially sailplanes, having the elevator circuit free of slop and friction is extremely important for good feel and handling. When you spend a lot of time circling, and also when you need to smoothly transition along a wide range of speeds, it's very important that you can feel what's going on and react to it easily. A very low-friction elevator circuit is one of the things that makes the Vans RV airplanes so much fun to fly. The aileron circuit is important too, but not to the same degree as the elevator. Rudder is much less important, as are secondary systems like flaps and airbrakes.
The issue with sailplanes is that there are rarely just one or two slide bearings. For the 5/8" push-pull tube that drives my elevator, there are seven guides along its length between the cockpit and the vertical fin--five in the aft fuselage, one under the drag crossmember, and one more where the push-pull tube goes through the seat back bulkhead. In the wings there are three guides for the tube that connects the inboard and outboard flaperon bellcranks. With just straight bushings, the accumulated friction would be quite noticeable and unpleasant. With the linear ball bearings, my elevator push-pull tube will move downhill under its own weight with a slope of only 2 degrees.
The V-tailed Schreder HP-gliders are especially prone to suboptimal handling due to friction from the hardening of the nylon slide bushings in the aft fuselage. Each ruddervator tube has six nylon guides, and in order to make a pitch input you have to move both tubes--that's a dozen guides worth of friction to overcome.