comrades, I think economics (largely) backs the old left:
Law of Utility http://en.wikipedia.org/wiki/U
The left amongst other things, is firmly in favour of redistribution, to limit the gap in earnings. In absence of a proper leftist foundation to our political economy - a large and growing amount of money in the economy becomes spent by a few people who see diminishing utility of their rising wealth.
And this is the problem:
As the rich get richer, they will value the money they gain less and less, and so spend it more frivolously. As our entire economy sees consumers spending less and less wisely efficiency of the market must drop.
Net GDP/capita falls below its potential as the gap widens. And this is born out in comparative stats between nations.
GDP/capita vs GINI