How is losing an ability part of evolution?

Alright, so it's believed the appendix and one point helped in our disgestion of dietary fibers, and we could consume grass/bark/plants like heribovores. So how is losing that ability evolutionary?

Why don't we see more mutations?  Sure there are deformities and retardations, but rarely is there some mutation that screams "holy shit we need to procreate and pass that gene on."  For example, a human who could live normally with horns, wings, and a tail.