The point is not for quick shifting. The point is that full suspension bikes (of non-URT design)make for pretty awkward singlespeed bikes, as chain growth when traversing through the suspension travel can cause big problems, especially if it's anything with a decent amount of travel. Most SS chain tensioners aren't designed for that sort of thing...deraillers are. This way allows you the versatility of being able to select different gears for different terrains/areas but like he said, without the cable making barspins or tailwhips impossible/annoying. In this style of riding, gear changes are infrequent anyway.
fair point but why not run single speed with a chain tensioner? that means less weight and less maintenece but then again you do loose the versitility of gears