The ironic thing is that Arlington is a very Catholic town, and when the church decides to do something truly Christian with its money, some residents reflexively fear that a relatively dense development of multi-family dwellings which are elgible for subsidies is going to bring in the wrong element. With the fall of rent control and growing income disparity between blue- and white-collar workers, new purpose-built affordable housing is the best way to ensure that not only people pulling down salaries in the low six figures can afford to live in a nice place near a city.
And, to put it bluntly, affordable housing is in the interest of affluent people as well. As people push out from the city, older suburbs (like Arlington), and even newer 60s suburbs to "escape" their neighbors and the inner-city problems that they think cannot take root in the exurbs, the poorer people who are willing to take low-paying jobs (which are everywhere, including the suburbs) still need a place to live. If they can't live near the places they can find jobs, and they can't afford a car, they must rely on our deteriorating public transportation system or look for work in the inner city. But despite the revitalization of cities like Boston, most job growth happens outside cities.
Another irony is that many of the 300 people who attended that meeting in the basement of St. Camillus could soon find themselves pushed out of the Arlington real estate market if present trends continue (and prices even now are only mildly affected by the dotcom hangover). Well, maybe you do get you deserve.