To take the definition of the CSS font-family property " This property specifies a prioritized list of font family names or generic family names" [1].
Thus if this change puts a free font first we are thus saying "supporting free fonts is our priority". I'm not sure any other big sites do this and I think that in itself is a big fricking deal.
I think regardless of install base and bugs in that free font we send out a message that might give a lot of attention to free fonts and be a positive thing for FOSS in general. The font might be downloaded more and that font might be packaged in OS installs as a result of this.
Restricting ourselves to only free fonts seems like cutting your nose off to spite your face. As Steven points out we have to think about the world we live in and that we want our content to be as readable as possible (as we want to imagine a world in which every single human being can freely share in the sum of all knowledge and to share it helping them read it plays a big part).
[1] http://www.w3.org/TR/css3-fonts