You people have people whose job it is to criticize the government?
That could make a ton of jobs here
(Neat idea, but how does it usually end up working?)
From my (distant but interested observer) observation, it works pretty well. Because each department has its own expert critic, so government waste and outlandish decisions tend to get noticed and railed about pretty quick. BUT, because it's a Shadow Minister's job to complain about everything that gets done, the public tends to take it with a much bigger grain of salt than they do in the US when the opposition party rants about "gub'mint done evil".
There's also the fact that when government inevitably switches hands, you've had a chance to see how various MPs performed and you have ready-made candidates for the real Cabinet posts.
I was lucky enough to host an Australian MP for an afternoon several years back, and he was a Shadow Minister (at that time) under the Howard government. When Labor won in 2007, he became a Minister in the Rudd government with the same portfolio he had been "shadowing" for years. We had some good conversations about the Australian political system vs. the American system, especially given the fact that Congress at the time was essentially a rubber-stamp. One point that he made was that an institutionalized system of dissent like the Shadow Cabinets tends to blunt the use of charges of "being unpatriotic" to quash dissent from the opposition party, like it was being used after 9/11 and especially in the run-up to the Iraq War.