The problem is that there are more than two buckets. Most MailChimp type emails are not spam, but they would probably fall in the bacn category. People generally don't want to unsubscribe, but they also don't need their day interrupted in order to see the latest marketing material as soon as it arrives. That is the purpose of the promotions tab. You can ignore those semi-relevant emails until you are ready to look at them.
... and I find one effect of this (very nice, and seemingly quite accurate) categorization on me is that I actually end up viewing "promotional" emails in a more positive light.
Now they're not being shoved in my face, not obscuring personal email (which used to be a problem with the old single inbox: I'd miss email from my mom amongst a bunch of random low-value emails), and not sending me annoying alerts on my phone. I don't have to skip past reams of "promotions" when I want to open that email from a friend I got a few days ago. I don't think I see, or open, or "open" promotional email any more or less often than before, but now that it's more on my terms, my mood when I do so is better.
For companies that use promotional email in an honest and straight-forward manner, I suspect the net result of this change may be positive... it's not just about "opens", it's also about the impression your email makes and how that effects peoples' attitude towards your company/service.
