Bear with me on this- but I don’t think this sort of a demand is a crock, per se, although I do think that it is trotted out too often in debates around here. In my opinion demanding that someone back up their rhetoric with their actions is entirely reasonable. Ex: If someone goes around decrying U.S. dependence on fossil fuels while driving a 3 ton SUV, then this person is a hypocrite, and I don’t think anyone would fault me for asking them to pick a stance and abide by it.
That being said, these sort of demands are made far too quickly, IMO. They are often unleased at the first inkling of someone not practicing what they preach, even if this inference on the part of the accuser is flawed. I think it is a knee-jerk reaction that should be bridled in deference to more civilized forms of debate, especially if the points of an opponents stance aren’t well understood. Also, the demands made are often unreasonable considering that most people’s views aren’t strictly black and white. When someone comes out in favor of universal healthcare (or whatever) they aren’t necessarily echoing Patrick Henry (‘Give me universal healthcare or give me death’), they are merely saying that they believe this is a good idea and possibly implying that they would be willing to sacrifice something (NOT everything) in order to bring it to fruition.
Ultimately, however, the demand for someone to put their ideas into practice is a valid one, but if/when someone issues this challenge, they need to be sure it is commensurate with the stated ideas, and not some extreme, unattainable demand meant only to undermine their opponents credibility.