I wouldn't say that economics is an illegitimate field without useful findings, but it may well be underserved. It contains a lot of elegant mathematical superstructure build on shaky foundations. Treating normative theories like Von Neumann-Morgenstern utility as if they were descriptive is of very limited ... utility. As Daniel Hausman, cited by Leiter, wrote,

[T]he justification for a particular paradigm or research program, like the justification for the commitment to economics as a separate science, is success and progress, including especially empirical success and progress. Since economics has not been very successful and has not made much empirical progress, economists should be exploring alternatives. . ..[U]nless equilibrium theory [the core of what makes economics a distinct science according to Hausman] has captured the major causes of economic phenomena, the separate science of economics can never be successful. If, as seems likely to me, there are systematic failings of human rationality, and economic behavior is significantly influenced by many motive forces, apart from consumerism and diminishing marginal rates of substitution, then equilibrium theory is not a very good theory, whether or not there is anything better. ...

Many (behavioral, especially) economists have proposed partial models accounting for a given irrationality or deviance from standard choice theory. What I haven't heard of - and admittedly I don't follow the subject closely - is any synthesis that covers a wide range of real human choice patterns.