Higher marginal tax rates increases the $ value of mortgage interest deduction for the same amount of mortgage interest payment (since a deduction reduces taxable income, but the amount of reduction in actual tax liability is the reduction in taxable income times the marginal tax rate.)

I see, but most mortgage doesn't have interest income tax deduction here in Canada?

edit: Just check interweb, only mortgage that being used to invest for more incomes are eligible for tax reduction in Canada, thus more trouble to convert a house mortgage to investment mortgage.