Check out
this thread for many pictures of 'omniwheels.' In the
white papers, you can find this AutoCAD drawing of team 226's crab module. The wheel they used is an omniwheel.
Any system that uses skid steering wrestles against friction. Even if you're rotating about a point, with one side in forward and the other in reverse, the wheels are still sliding perpendicular to their intended motion (i.e., not forward or backward). This creates friction. Belted tank drives exhibit the same problem, but there's more surface sliding along the floor, potentially making it worse. (See
this thread for a discussion on how surface contact could impact robot performance.)
Many teams choose to live with these losses. It's a balancing act, though, because by adding additional load to the motors while turning, the current draw increases, and you risk tripping a circuit breaker.
Teams choose to minimize these losses in a few different ways. Omniwheels are popular. Casters are popular also, but aren't powered, traditionally. There are some issues with simply strapping a gearbox onto a freely rotating caster. Crab modules are another answer. These are, essentially, powered, controlled casters. They're considerably complicated to build. Tanks can pivot on a single point if the system is designed propely.
Hope that helps.