The main problem is that retirement income is often heavily driven by one spouse, meaning most of the taxable income falls into one person's hands. This leads to paying more tax than if the income were split, due to Canada's progressive tax system.
This video explains a common pitfall for couples retiring before age 65: an unequal distribution of retirement income, leading to higher taxes. It demonstrates how using a spousal RRSP can help equalize income and reduce the tax burden during the crucial pre-65 retirement years. The video also touches upon the attribution rules associated with spousal RRSPs.