You don't "have" to do anything ;) and a lot of people opt not to do enemas or colonics. However, imagine having a clogged pipe. Wouldn't you want to send some water through it to flush it out? And if you have a skin abrasion, you have to clean it before it can heal properly, right? :)
As for natural, it's much more natural than pills! Enemas have been the basis of healing for centuries. Ayurvedic medicine always includes enemas in healing and rejuvenation. Our great-great-grandmothers swore by them. It wasn't until we got more dependent on the AMA type physicians rather than our own families, herbalists and mid-wives that they went out of fashion. And those guys thought leeches were better!!!
Doing something is better than nothing, so if all you want to do is P&B shakes then just do that but make sure to do them thoroughly. They will definitely benefit you! :)