Those are clearly "special" emails, so I don't think it can be safely assumed they would be processed the same way as normal emails - it would have to be tested.
Personally, I think Google added the Promotions tab exactly because they knew they would get a major backlash against putting ads in the normal inbox.
Interesting. That could mean that this is a stop-gap before ads appear in the main inbox for users who don't use tabs. I expect that would take years though as people would need to be conditioned to the current system first. In any case it feels like a slightly underhanded way to 'subscribe' you to 'mail' you didn't ask for.
I use Gmail over IMAP so no of the web UI stuff they've done has affected me at all (in aggregate, I feel that's worked in my favour).
Personally, I think Google added the Promotions tab exactly because they knew they would get a major backlash against putting ads in the normal inbox.