I think it's a little unfair to use Medway and Holliston as examples of greater Boston suburban density. Those two are seriously ex-urban (is that the phrase we're supposed to use now?). Although I'm not 100% familiar with every town within the 128 corridor, I'd say that they are all, more or less, as dense as they could be before we'd start to call them urban. Furthermore, there are a few towns in there, like Salem for one, that are really pushing what you'd define as suburban/urban (I'd lean towards urban). Now, getting out towards 495 where Holliston and Medway are, sure, things drop off quite a bit. But even then we have a few pockets of urban density in places like Lowell, Lawrence, Framingham, and Brockton.
Of course, this entire debate is pretty easily answered if we had some good density data. I'll see if I can dig up some good info on the Google.