Actually, throwing a spear has some very definate advantages. It's been proven that humans are hardwired for throwing stuff. By throwing a spear at targets (stationary and moving) we not only strengthen the parts of our brains that control eye/hand coordination but also our problem solving and predictive skills. There's a lot of physics and geometry involved in throwing a spear which the thrower can actually see clearly (velocity, parabolic arcs and the such). Shooting a gun doesn't give the same "deep brain" workout as it happens to quickly for the brain to follow.

-Blast