The chlorophyll does not contain all the vitamins and minerals that the whole green juice/powder contains. Chlorophyll by itself is beneficial, but is only one component of a green plant (juice/powder).
Most people drink green juices/powders for the enzymes, vitamins and minerals in those plants, not just for the chlorophyll. Of course, you get a lot of it if you drink green juices, so you would only need to take more chlorophyll if you have a specific need for it (such as Body Odor , for which many people take it).
Think of it this way: it's the same question as, why would you drink fruit/vegetable juice (which contains vit. C) instead of taking vitamin C separately. Most people prefer the whole nutrients of the juice rather than isolated vitamin C. Of course, if you needed mega doses of vit. C you would have to add a vit. C supplement.