That one guy that works on GIMP.
He could take the 2.6 source, work on his own machine and/or with collaborators, without any of them telling anyone what they're doing, without any of them releasing any source to the outside world. When Martin's happy he can release 2.8 version.
It would still be open source.
There is no requirement for him to release incremental versions.
If he only made available the source at each major release, I could not go and get a snapshot of the source now and build it, but he would still be working within the open source model.
The other side of the coin: Microsoft could suddenly decide to start doing nightly IE10 preview releases.
They don't need to release the source to make it possible for anyone to go get the latest snapshop and installing it right now.
And the odds of Microsoft ever releasing IE as open source is slim.
So going back to this:
quote: you
You can install it currently if you like because it's open source.
The reason I can install it currently is
NOT because it is open source.
It's because the developers have chosen to make it possible for others to get the latest version of the software.
Doing this
makes sense for open source but is not required by open source, and nor is being open source a pre-requisite for doing it.