What they ignore, however, is that it is rarely the case that a plan can be executed precisely as envisioned.
Chess players and computer security professionals learn a hard lesson early in their careers: you must assume that your adversary will behave intelligently, not that he will behave stupidly. You must judge your plans not against what your wildest dreams, but against what will happen if a smart opponent attempts to thwart your actions.
Similarly, when judging the proposal that the government undertake some action, one must consider what will happen if real-world bureaucrats, not saintly geniuses, execute the plan, and what will happen if an array of real world forces interfere with it.
For example, consider the dream a number of neo-conservatives had when they dreamed up the idea that a strong U.S. should re-shape the Middle East by invading selected countries and imposing democracy by force majeur. (This isn't a conspiracy theory — the idea was written about in public even before the 2000 elections.)
Now, it is all fine and well to daydream about our military might sweeping aside dictatorships without loss of life, and of crowds of cheering people, freed of decades of tyranny, greeting us with bouquets of flowers in the streets, and immediately setting up Western style democracies.
However, in the real world, we have a military that is not run or staffed exclusively by saintly geniuses. Opponents are also unlikely to cooperate with our plans — they will seek the most effective possible means to thwart us, and sometimes, they'll be able to find such strategies.
We must therefore not judge plans against our hopes and dreams, but against what is likely to happen in the real world. Indeed, the prudent planner judges a plan not only against the best case scenario but against a worst case scenario, because sometimes the worst case, not the best case, is what happens.
When examining a proposed government action, we must be especially skeptical, since there is no mechanism that will act as a check on poor performance. In the free market, companies that fail to meet their customer's needs go bankrupt, but governments are funded by taxation and have no such limitation. A CEO can claim in public all he likes that he was not responsible for "unforeseen circumstances" but pleading will not save his company from dissolution. If, however, a military commander's mistakes result in massive deaths, or if a bureaucrat's mistakes result in vast waste and the failure of a program, it is unlikely that they will be punished or that their work will be terminated. Instead, if they argue well, they might even get additional resources committed. In the commercial world, the best run organizations get more resources with time, and the worst run disappear. In government, the most politically astute organizations get more resources with time, and often especially if they have failed at their missions, while the best run organizations have no particular mechanism that rewards them or increases their scope.
This is the reason that you rarely wait for long on line at the supermarket, and it usually has what you want in stock. This is also the reason that you can wait interminably at the DMV or a similar government office, only to be told that you have to come back with additional forms the next day.
The next time someone says to you "wouldn't it be great if the government enacted my pet idea...", ask yourself what would happen in the real world if the government attempted to execute "the perfect plan", and not what would happen in the word of one's fondest dreams. In the end, the government will not do you want; it will instead do what the political process permits.