In other words, you needed to set automatic criteria that determined what messages would go into that bundle before you can enable the bundled in the inbox feature. Well not that seems to be no longer a requirement, but why? If you are not setting criteria to place emails into a bundle, then how would new emails cause the bundle to show in the inbox? Excatly, it cant.
Regardless, it is my public duty to report all features found in Inbox, whether they make sense or not. But seriously folks, how are you using the bundle in the inbox without bundling rules?