I would think so, because the purpose of WITB originally—and, as I understand, the earned income tax credit south of the border—was not so much redistributional as it was a work incentive to help people get over the welfare wall, to ensure that wage increases and increased hours always pay more. The phase-out of some of these benefits has the opposite effect at certain income levels, as you've confirmed.
On the same subject of marginal effective tax rates, under the changes to the Canadian-controlled private corporations, when a company's passive income goes from $50,000 to $50,001, what will be the marginal effective tax rate on that additional dollar of passive income?