My immediate thought is whether researchers or investigative journalists will find cold hard US government backdoors. This is potentially big.
If it really does exist, the backdoor can simply be inserted during the last-minute compile before release. It would be invisible in the code repository, the vast majority of internal developers at Microsoft won't even see anything unusual. Also, I heard anecdotes that Microsoft already allowed governments to audit the source code of Windows under NDA on multiple occasions in the past. But if you cannot guarantee the…
I hate it. It was so easy in DOS era - same source, same binary file, easy peasy.