It's here for download https://support.apple.com/kb/DL1804?locale=en_GB
Err Duh, that is an update from 10.10.0 to 10.10.3 not 10.6.8 to 10.10.3.
Yeah, having patches for the older OSes would be nice too, at least for the ones they say are "supported".
As for babysitting a GUI app. I run Linux on this machine 99.9% of the time. That is what most of my applications I use run on, and what I'm most comfortable with.
Occasionally there are tasks that require MacOS X, and for that I dual-boot.
I cannot run the Apple App store from the Linux environment and have it update my MacOS X installation (and lets face it, it'd be unrealistic to expect this), and so to update MacOS X, I have to be running MacOS X at that time, which means I cannot be doing what I'd normally be doing with the computer.
Ergo, I'd be stuck with babysitting the app while it does the OS updates as the applications I'd more likely want to use are on Linux and not MacOS X.