There is no benefit to society and "the Progress of Science and useful Arts" if somebody has just an idea (that anybody else could have too - maybe about a cool device, a cool algorithm, a cool user interface), publishes this idea and gets "the exclusive Right to their respective Writings and Discoveries."
That's just it, though: they don't get legal protection for publishing the idea. They get legal protection for patenting it, which requires that the idea be more than an idea--it has to be an invention. The USPTO is charged with the task of ensuring that the invention is novel, non-obvious, useful, and adequately reduced to practice. Basically, they're giving you an exclusive right for giving someone "instructions" on how to practice your invention. Whether the USPTO does a good job at this is obviously the issue, and my answer is that they don't, which comports with your overall thesis.
That's just it, though: they don't get legal protection for publishing the idea. They get legal protection for patenting it, which requires that the idea be more than an idea--it has to be an invention. The USPTO is charged with the task of ensuring that the invention is novel, non-obvious, useful, and adequately reduced to practice. Basically, they're giving you an exclusive right for giving someone "instructions" on how to practice your invention. Whether the USPTO does a good job at this is obviously the issue, and my answer is that they don't, which comports with your overall thesis.