Nay
If you want an explanation - here we go: I disagree with your arguments that said act would be just for "Protectionist construction unions", instead I say it was created to ensure that workers are treated decently. I believe that it is common sense that we pay workers on government construction works the local prevailing wage, simply to allow these workers a decent standard of life in the local area. Even if we found "cheaper" workers from other areas, we should still pay them the local wage, not the "prevailing" wage in their area, as costs of living in areas with higher wages are usually higher than in areas with lower wages. Explanation enough?
First of all, the question of why it was created in the first place is not up for dispute. It's historical fact that racism was the original motivator (a simple Google search will confirm this).
But, as for the contemporary issues with the law, just because a certain amount is a "prevailing wage" in a given area does not mean everyone in that area is paid that wage. Additionally, many contractors bring in hire workers from other areas (Davis-Bacon was originally proposed after an Alabama contractor attempted to bring black workers from the South to build a hospital in Long Island). Forcing these contractors to pay the prevailing wage for the specific area in which they are building something amounts to protectionism against workers from other places (the same logic could be used to justify immigration restrictions). Finally, say you have two workers who apply for the same job, one of whom is more skilled than the other. If the employer they're applying to is required to pay them a certain wage rate, he will of course choose the higher-skilled one. When employers can hire lower-skilled workers at lower wage rates than high-skilled ones, it actually makes sense for them to hire them, but when the government takes away that option (as Davis-Bacon does for Federal contractors), they have no reason to ever hire low-skilled workers. This is especially pernicious because it prevents said low-skilled workers from acquiring skills to begin with by working, thus trapping them in permanent unemployment.