To see the force of this question, consider the following:
- Conventional economics tells us that austerity depresses output at the zero bound.
- More open borders would not make British workers worse off; this is factor price equalization.
- Thousands of high-paid financiers are overpaid charlatans; this is the implication of the efficienct market hypothesis, which says that stock-pickers can't beat the market in risk-adjusted terms.
- The diminishing marginal utility of wealth implies that progressive taxation is welfare-enhancing.
- Drug legalization would improve the lot of young black men; this follows from the presumption that markets work less badly than state interventions.
- My scepticism about managerialist hierarchies owes more to Hayek's point about the limitations of centralized knowledge and control than it does to my Marxism.
All these points imply that you can adopt a position well to the left of the Labour party whilst being a quite orthodox economist. You can be left-wing whilst being wholly ignorant of Marx, Kalecki, Sraffa or Minsky. (Equally, you can be heterodox and right-wing, as some Austrians are.)
This is not necessarily to agree with Noah that economics in general is on the left, although many of the authors of orthodox economics such as Arrow, Solow, Tobin and Samuelson were. It's just to say that there's no tension between orthodox economics and leftism.
Which prompts the question: why do so many on the left feel the need for heterodox economics?
One perfectly good answer is that there is, in fact, a lot wrong with the orthodoxy. My personal beefs would be, among others, that: representative agent models eliminate important network effects; that preferences aren't always exogenous and well-defined; that the idea that wages equal marginal product is deeply dubious; and that power and exploitation are serious issues.
I fear, though, that there might be less respectable reasons why some lefties are so quick to reject economic orthdoxy.
One is an ignorance of what the orthodoxy is; there's a tendency to equate economics with the view that markets always work - when in fact, as Simon says, "mainstream economics is all about market failure". (My view is that whether markets work or not is an empirical matter in which precise institutional details are crucial.)
Another is a form of halo effect. People who are radical in political often want to be radical in other ways: they tend to use "edgy" and "transgressive" as terms of commendation. This leads to an interest in heterodox ideas.
What I'm trying to do here is to take a little heat out of the orthodox vs heterodox debate. This should not be regarded as a necessarily left vs right political matter.