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).