I think the problem with blending wheatgrass is that the fiber is indigestible by we humans. Other foods, though, don't have that problem.
Have you read "Green for Life" by Victoria Boutenko? She promotes blending over juicing--and the use of 'green drinks' in particular-- because blended foods contain their fiber.
Those who promote juicing over blending say that removing the fiber is a benefit, because it allows the nutrients to be absorbed more directly. If you have non-organic produce, juicing is probably better, because the pesticides, etc. remain in the pulp.
Also, with juicing, you get a more concentrated food: good because it allows you to consume more of the vitamins and minerals, but it can be bad with some high-sugar foods like apples, for example, because you are removing the fiber that helps slow down digestion of the sugars.
One drawback to juicing, then, if you do it too much, is you could lack fiber in your diet. But if you don't do it to excess, and make sure to have enough whole foods along with it, that shouldn't be a problem.
I think it's a bit "6 of one, 1/2 dozen of the other," meaning that both have their benefits.