On the bridge, so officers can look cool. No way the "we have first backup, but what about second backup" federation doesn't have redundant, localized inertia damping in the cargo bays.
Yeah, but after you're ultimately responsible for breaking the spine of the Klingon officer who almost certainly outranks you, you make sure that cargo doesn't budge even if the ship does a barrel roll without dampeners and gravity going on and off.
Yeah, but they're maneuvering at appreciable fractions of the speed of light when they travel at impulse, so there's no way anyone survives if they're all offline. They have to mean that the primaries are offline, or they're offline in certain sections, or something like that.
Can't possibly be the bridge as a whole. When I said "certain sections," I meant, like, every third square centimeter or something. If the entire bridge inertial dampeners were completely offline, our heroic crew wouldn't be jumping madly across the bridge, they'd be a thin paste of organic material on the wall.
Actually, having individual sections fluctuating could explain some of the wilder dives Kirk & Co did during battle sequences, now that I think about it.