JW Mason channels my favorite economics professor, Austan Goolsbee, on investment tax credits. According to Goolsbee's early work, investment tax credits produce surprisingly little extra investment. Instead, the benefit accrues in the form of "rents" (economic windfalls) to producers of capital goods. Since in the short term, supply is constrained, the prices of capital goods are bid up, generating a windfall for makers of capital goods, but little bang for your buck in terms of actual productivity enhancement (h/t Rortybomb):
From his Investment Tax Incentives, Prices, and the Supply of Capital Goods:
Although there appears to be an abiding faith among policy makers that tax incentives can influence the investment decisions of firms and serve as a tool for stabilizing the economy, empirical evidence for the connection is weak. Econometric research has commonly found that tax policy and the cost of capital have little effect on real investment. Economic theory predicts that the marginal user cost of capital should be the primary determinant of investment demand but actual estimates of the price elasticity of nvestment ... mostly lie between zero and -0.4... The evidence that investment is only modestly responsive to price has been one of the most robust findings of the empirical investment literature...
In addition to their large revenue costs, investment tax subsidies may give large, unintended rents to capital suppliers without increasing real investment until several years later because of the short-run asset price responses of capital goods. For policy makers interested in using tax policy to stimulate investment or, especially, to smooth business cycle fluctuations, the results are not promising.