This debate over affordable housing in the Lower Hill site is one of the more important public policy issues surrounding development in the City in the near future:
http://triblive.com/news/allegheny/4819865-74/hill-housing-penguins#axzz2h2UHYLt2
Just to give my two cents: housing affordable to low-income households in convenient locations (with good access to major job centers and other regional amenities) is a worthy goal. That said, so is housing affordable to middle-income households in convenient locations. Generally, the big picture is that if you don't build enough units for middle- and higher-income households, they will end up finding a way to drive out lower-income households in all but the most undesirable of urban locations.
Given that background, I'd like to see this debate move past the current framing, where the number of units is considered fixed and the issues are what percentage of those units will be affordable at what income levels. Instead, I'd like to see affordable housing advocates focus on increasing the total number of low-income affordable units without subtracting from the middle- and higher-income units, meaning they would be de facto advocating for more units. And if anything, that should make the financial numbers work out better for the developers for a given number of affordable units (since the marginal cost of additional units should be relatively low).