How much will change now that the FHA backs mixed-use projects?

Matt Yglesias celebrated a rule change in which the FHA allows a greater percentage of commercial space in supportable developments. Reading into the details as supplied by the New York Times, however, made me question to what extent the new rules will actually impact development.

First, the article states that the previous rules were not much enforced before 2007, so why weren't there more mixed-use developments before 2007? Second, the new rule increases the allowable footprint of commercial space from 25% of the development to 35%. 35% surely allows more flexibility, but it seems that a 25% commercial footprint would be reasonable in most developments. So why weren't there more mixed-use developments with up to 25% commercial space even with the old rule? Most importantly, there are still other considerations such as zoning, expected buyer preferences, and developer experience that will change slowly over time before mixed-use development can be a significant portion of all development.

Mixed-Use Development, so risky!
Source: PhillyShark

Leaving aside the discussion of to what extent the federal government should subsidize homeownership, which Yglesias brushes aside, I would classify the rule change as a necessary but not sufficient step on the road to greater environmental sustainability in our cities' structures.