In my last post at OL, I touched indirectly at this topic, but couldn't work in a paragraph or two about it without straining an already long enough post. The problem has bothered me for awhile, and it flares up every time some Villager dies or retires and they're all falling over themselves to praise that person for their centrism and moderation, as if these are self evident virtues.
As I see it, the problem with centrism is similar, but worse than the problem conservatives have in defining their beliefs, because centrism has even less meaning to it than conservatism. Not only is it bereft of meaning in itself, but any meaning it defines is dependent on liberalism and conservatism as the "extremes" which it seeks to place itself between. As I demonstrated in my post on conservatives' own flailing attempts to define conservatism, now imagine trying to define an ideology that relies on another incoherent ideology for meaning. It would be stupid to define an ideology that was, say, the opposite of objectivism or marxism - the results would not likely be something anyone could really intellectually support, but at least you wouldn't have difficulty understanding what this anti-belief of those two meant. With centrism, you have one foot in liberalism, which at least can be understood to mean something but what is the second foot standing in?
Beyond this, if you still wish to claim that conservatism is coherent or no worse than liberalism then you have the very serious problem for centrists in asking how they "split the difference" on subjects which do not have mid-points. It isn't a black or white world, but on many issues there isn't really any kind of useful middle ground. Either abortion is legal or it is not. Either same sex partners can get married or they cannot. Yes there are psuedo compromises on those issues like carving out exceptions on abortion for rape, incest or health of the mother, but those really are themselves generally incoherent. If a foetus is a human being with a right to life, why does the mother's lack of consent or relation to the father matter? I realize a great many "pro-life" people would support allowing abortions in those cases, and pro-choicers should use that as they can to defend choice, but it does demonstrate that the political beliefs of many people are not thought through and consistent. Perhaps no one is 100% consistent in their beliefs, but certainly the degree matters a great deal.
Where centrism makes some kind of sense is on issues involving numbers, because those are the most easy to split the difference on. Liberals ask for 100 of something, conservatives want 0, maybe we settle on 50 (or in America, more likely today we settle in the "middle" at 10). The problem here is that this only works where input and results have a linear relationship, (where spending twice as much gets you twice the amount of the desired thing). Many things in real life do not work this way. The stimulus was a great shining example of a chasm that could not be lept in two jumps. Another one is global warming. We as a species must reduce our greenhouse gas emissions below a certain level by a certain time or we risk natural tipping points making the process irreversible. So when liberals ask for 350ppm, centrism might tell us to settle on 400ppm, but if 400ppm still results in the Siberian tundra melting and releasing vast quantities of methane, we'll still end up at a world 4-6C warmer by 2100 with all the fun and excitment that entails for all concerned.
Even something like progressive income taxation cannot really be addressed by centrism. One of the goals of progressive taxation is to flatten the wealth curve and ensure a more equal society. At any given time in a society, that society is either becomming more equal, more inequal or just possibly staying flat in terms of income distribution disparity growth. If we are debating changing the tax structure, we cannot arrive at a policy recommendation in terms of numbers without understanding first what is our goal? Do we want a more equal society or a less equal one? Is the current structure leading to a more or less equal society? A coherent form of centrism might seek to moderate either conservatives or liberals in the structures they propose to bring about their competing goals, but really centrists must pick a side here and actually lack any ideological basis to do so. Do they want a more equal society? I have no idea.
More can and should be written about this, but I just wanted to highlight the unspoken problem we observe when anyone praises Snowe or Nelson or Lieberman for being centrists. Moreover you do see individuals proudly describing themselves as centrists and I don't think they have any sense how absurd this is.