First, this line gets said a lot.
Is there any evidence to suggest that its true? There are plenty of examples in the city where this didn't happen. Downsview, much of Sheppard outside of North York Center, much of the Danforth. Really the only example of a subway encouraging density was north york center, and I suppose Yonge and Eglinton now, both of which have the "On Yonge Street" advantage.
If you look at Mississauga, they've encouraged density long before they got any form of rapid transit, and the rapid transit they did have planned was an LRT, not a subway. In Ontario thats probably the best comparison considering both are trying to build density around a mall.
Subways are not a guarantee of density. Nor are they a requirement of density. Proper urban planning is what brings density. You need to create favorable conditions and actually have a conscious plan of what you want to achieve. Zoning, tax-incentives, and yes, infrastructure, its all relevant and it all goes together. People here seem think that the one single way develop is possible is a subway but there isn't any reason to believe that is actually true.
Secondly, why does anyone care? Seriously, why does the average person in Scarborough care at all about development around the STC? Sure, the city should probably care (though building subways through green fields hasn't stopped them before...), but why do you? It seems completely irrelevant to the average person's life. The only thing that should be important is how long it takes to get to work, and at last I checked the benefits would be a couple minutes at most overall, and more time spent on a bus.
"Build a subway to encourage density." Thats a pretty damn expensive way to do it. How about we "build a subway to improve the efficiency of our transportation network so it doesn't take people two hours to get home."