Sorry, you read wrong. The grip isn't required for sealing. In fact, the opposite - the body has reasonable sealing, the grip itself has some sealing, but there is no sealing where the grip attaches to the body (only gravity keeps water from from flowing up into the battery compartment. Well it's pretty much 95% sealed battery case and in this case I'm talking about Canon BG-E7 not aftermarket battery grips.
Only part what isn't sealed is plug.
Really? So...you'd be perfectly happy if the roof of your house was 'pretty much 95% sealed' because if 5% of it leaks, that's ok? The only unsealed part is the plug? Whew, that's a relief, because there's probably no need for sealing of an electrical contact that provides power to the camera, sure, seal everything else, but let water and electricity mix, they play so nicely together. That's like saying it's best if the hole in your roof is right over your fuse box.
Seriously, though, I think this is a design flaw on Canon's part. When I remove the grip (yes, the Canon BG-E7) where the battery compartment opening connects to the grip is just plastic on plastic. The battery compartment does have sealing, but the foam-rubber seal is on the battery door itself, and of course that's removed and stored in the slot on the grip when the grip is installed, meaning no effective sealing. Canon could have replicated the foam-rubber seal on the grip, but they didn't.
FWIW, when I go out in the rain with the 7D, I remove the grip. That's especailly important if the camera is being carried upside down (e.g. from a Blackrapid strap), since water would flow down the camera and in between the grip and body, then flow down into the battery compartment when the camera was turned over.