This would be like if I worked on the core Windows SDKs and I could routinely test my changes against everything from Microsoft Flight Simulator to the Bing server code before I submit.
Because the build server is centralized it can be aggressive about caching intermediate build steps. Incremental builds aren't just incremental for you, but incremental for everybody.
You could. It would just not leave your branch for a while. Around the scheduled merges it would run against the tests of progressively more of the larger organization.
Parts of this actually constituted a good way to prevent being distracted by the bugs of faraway teams. If something reached your branch, where you were working, it was vetted by the tests required to make it into winmain.
The downside was that people got fairly political about what goes into the branch and when, even for small things.
Googlers like to joke internally that Google looks like a race car from the outside and like Moving Castle from Hayao Miyazaki'a cartoon from the inside, but that's not the case at all. Comparatively speaking it's a race car inside and out, it's just that the insiders don't know how shitty things are elsewhere.
P.S. I heard Bing is different, but I have no visibility into it, so can't comment.
At the time, Azure was a joke (partly due to the fact that the initial teams were headed up by ex Office devs with no cloud experience, if I remember correctly). But Azure was cannibalizing the Bing team pretty hard. I hear that strategy worked and that Azure is in much more capable hands now.
