Quote from Stats Canada on the use of low income cut-off figures (found here):

"Statistics Canada produces two sets of low income cut-offs and their corresponding rates—those based on total income (i.e., income including government transfers, before the deduction of income taxes) and those based on after-tax income . . . Although both sets of low income cut-offs and rates continue to be available, Statistics Canada prefers the use of the after-tax measure.

The choice to highlight after-tax rates was made for two main reasons. First, income taxes and transfers are essentially two methods of income redistribution. The before-tax rates only partly reflect the entire redistributive impact of Canada's tax/transfer system because they include the effect of transfers but not the effect of income taxes. Second, since the purchase of necessities is made with after-tax dollars, it is logical to use people's after-tax income to draw conclusions about their overall economic well-being."

Paying income tax to the government and paying tuition to the university isn't the same thing, but I think a rough parallel could be drawn. If you want to put everything down into "income" and "expenses" columns, then yes, you're right, there's 23.5k in the grad student's "income" column per year. Since tuition is a mandatory, institutionalized expense (like income tax), if your goal is "to draw conclusions about [grad students'] overall economic well-being," it seems to make sense to follow Stats Can's model and look at after-tuition income.

As a side note, some universities talk about giving a "tuition waiver." The technicalities of how money changes hands may be different than at U. of T. but the effect is the same: get scholarship, do TA work, get $8000 worth of education, go home with $15,000 to pay rent etc. Would you consider a tuition waiver income? I've always thought of my funding package in terms of that model -- I get 15k plus the amount of tuition, and that amount for tuition comes off the top before I can go home with my money.