I juiced vegetables religiously for about 2 years. In conjunction with a good diet, I didn't really notice any benefits one way or the other, although I did get a few nasty migraines when I first started.
Vegetables are obviously excellent sources of vitamins and minerals, which we all need a bit more of as athletes. However, some of the health benefits of vegetables may come from their mildly toxic nutrients, which is similar in a general sense to how we benefit from exercise (physiological stress leading to beneficial adaptation).
Therefore, my concern with juicing (when done frequently) is that there might be a negative effect of an unnaturally high intake of vegetables, similar in concept (at a high level) to too much exercise leading to overtraining. I think this applies with the green powders as well, which tend to be highly concentrated just like juiced vegetables.
Unfortunately, there's not much, if any research on juicing, so it's hard to say with any confidence whether or not there is benefit. For someone already eating a good diet, I would say don't bother, or only do it periodically. But for someone who, for whatever reason, doesn't eat many vegetables, it might be a convenient way to improve the quality of their nutrient intake.