Gracchi writes an interesting post describing how the academy used to interact more with the outsiders.
There are clearly two halves to this equation. The academy's influence on the outside world and outsiders' influence on the academy. I'm going to consider these separately as I'm not sure that they have waxed and waned simultaneously.
Input from the academy in the outside world
I'm not sure that scientists and academics really do have less influence on the world than they used to. Gracchi's main example here is Sir Alan Sugar:
"Sir Alan Sugar's derision for academics on the Apprentice is well known- the worrying thing is that such a bigot is advising the Prime Minister of the day."
I think Sir Alan's words could be heard from the mouth of practical entrepreneurs troughout the ages. What distinguishes contemporary business is how rare that attitude has become. Alan Sugar is retiring and one of the last of his kind; the poorly educated practical businessman is a dying breed. As more senior managers have been to university they are increasingly comfortable making use of academic expertise.
Few manufacturing companies are run without scientific expertise, either internal or bought from outside in some way. The basic methods of the social sciences are used in all kinds of businesses to varying degrees of sophistication. In the political world we're always looking to enlist science to our various sides. Even in debates like abortion where science doesn't really have a lot to offer.
I'm not saying the academics have it all their way, they never have and shouldn't, but they've probably got more of a voice now than they ever have.
The outside world's input in the academy
It's here where I do think there has been a decline. Gracchi cites Machiavelli, Gibbon, Le Carre and Cicero. You could just as easily cite examples from the natural sciences such as Priestley or even Einstein. Amateurs have made serious contributions to academic thought. It's hard to think of many examples of such amateur scientists today.
I think this change is driven by a rise in the amount of commitment, effort and particularly resources that are necessary to engage with the academic debate. The bar to serious academic investigation has risen over time for a number of reasons:
1) The onward march of empiricism. An ever increasing tendency to try and quantify everything means that there is a need to assemble and analyse data. This requires a set of specialist skills that most people don't have.
Even if you do possess those skills, or have the time to learn, it also requires access to data and software. While there are enough sources of free data for an amateur to do some great work they are at a serious disadvantage (even supranational organisations like the OECD charge for many of their sources) and the software used to perform sophisticated analyses can cost thousands of pounds.
2) The academic firewall. The best way to get involved in the academic debate is to read journals. Unless you have access to a university library or can focus on a few key publications this is practically impossible. You would have to buy so many to get a good grasp of most important subjects that the cost would quickly run to hundreds of pounds. Unfortunately, the charge for most journals operates quite effectively as a cover charge keeping out the amateur.
3) The increasingly narrow focus to academic research. I think there has been a movement towards research in narrower areas. Few academics are looking at the 'big picture' these days and most choose a small area in which to try and shed some light. I have one friend who has just completed a PhD studying religion in Cromwell's army, another who is studying the campaigning methods of a single American Senator. This all has to be rather bewildering for an outsider.
It isn't good for any institution, even one as fine as the university, to operate without outside input. Outsiders can bring fresh ideas and new perspectives that improve everyone's work.
In America, in particular, there is something of a solution. In the social sciences think tanks support work at, or near, academic standards. Research and development units at the corporations and in the military undertake significant research at the applied end of the natural sciences (Microsoft's work on quantum computing and the development of the Internet at DARPA shows how advanced this can get). Outside organisations replace amateurs as external influences on universities.
In Britain we are less fortunate. A smaller portion of our GDP is spent by British firms on research and development but the biggest divide is in the think tanks. Anthony Browne, head of Policy Exchange, wrote an interesting article for the Spectator setting out the sheer difference in scale. I was in Washington recently and noticed much the same thing. He focusses on their influence, longevity and scale but I've always been struck by the intellectual depth of their work. This article, by Donald Kagan (who I believe is also responsible for the idea of the 'surge') is a great example. Whether or not you agree with it that short piece is an incredible example of the kind of work the American think tanks sponsor. British think tanks don't have the resources to produce the same sort of output.
All this creates a problem. We have lost amateur intellectuals without enough of a replacement. British intellectual life is definitely poorer for that loss.