Eh, releases are arbitrary points in the development cycle. There is new, stable, stuff in the dev trunk today, we just don't get to see it.
I don't see any difference between Google code-dumping (because that's all they do) "releases" of Android and iD Software open-sourcing old versions of idTech. iD is just more honest about it.
If you remove the community-based aspect from open source, there is not much of an advantage in it. This idea of regular code-drops seem just like a way of maintaining the minimum requirements to conform to the open source tag.
I don't see any difference between Google code-dumping (because that's all they do) "releases" of Android and iD Software open-sourcing old versions of idTech. iD is just more honest about it.