Of course, the working poor are a benefit to the country. Only question is, to whom goes the benefit? The fact that we don't value a clean environment in a hospital or in the bureau, orderly stuffed shelves in the supermarket, or our elderly mom kept fed and clean, or cheap lettuce, and thus won't pay a decent wage for any of that, doesn't mean they, and their work, are not a benefit.
Those who know the price of everything, and the value of nothing, are unlikely to realize that it's exactly undocumented immigrants, exploited and taken advantage of to the hilt, who hold a giant mirror right before our faces, for us to recognize who we are. Oh, and, also how careless and disrespectful of other humans we have become, while we all, in a variety of ways, benefit from both the working poor and undocumented immigrants without realizing, or caring for, it, or them, for that matter.