In fact, you don't even have to assume. A preliminary study from e-mail marketer MailChimp saw a measurable downtick in the open rates of e-mails sent to gmail users immediately after it was rolled out to a wider audience.
Digital strategy nonprofit the New Organizing Institute (NOI) recently sent out an e-mail comparing the tab's devastation to the Death Star:
Now some of you might love this new organization of your inbox, that's great! But many important advocacy emails (like this one from your friends here at NOI), could get lost in the commotion of all these new tabs -- silencing our voices like those of the poor souls on the planet Alderaan.
The message went on to suggest ways to stop their messages from being stuck in the promotions tab purgatory, like removing the tabs altogether, or dragging e-mails from their group to the primary inbox.
Of course, digital advocacy groups aren't the only ones trying desperately to get out of the the promotions tab. Clothes retailer The Gap also recently sent a plea to Gmail users to move their newsletter into the coveted primary tab.