The question is a really good one, and it goes to the heart of what is wrong with all of the hundred or so provisions that relate to family relationships in the Income Tax Act and other taxing provisions.
It's absolutely right that at the beginning of World War II, this was removed to get women into the paid workforce. At the end of World War II, this was reinstated, and the precursor to the Canada child tax benefit was put into place—the family allowance—to help women feel that they still had some money in their hands, to literally ease the political opposition to driving women out of paid work.
This dependent spouse credit is now larger than it has ever been. It is only available to families that have a high enough income that everyone can live on that one income. So it's really for a very relatively small proportion of the population who can take advantage of it.
It is a form of income splitting. It treats a woman as a tax shelter. It treats a woman as someone who can essentially be expected to do unpaid home-centred work that is untaxed and that adds value to the family, and it is itself one of the key mechanisms by which the tax system prevents women from engaging in paid work.
When a couple sits down to decide whether a woman should enter into paid work, one of the calculations that is done—women are very aware of how much tax they pay—is how much the loss of that dependent spouse credit is going to cost, in conjunction with the loss of the unpaid work the woman can do in the home and what she can earn.
It used to be, right after World War II, that Chatelaine was publishing articles showing why it didn't even pay a woman lawyer to go back to work, because the net after-tax profit to women was simply too low at the margins.
Definitely, if I could rewrite the Income Tax Act, this would be repealed, as it has been in many other countries and as it was slated to be in Canada some years ago, along with the dozens of other provisions that work exactly the same way.