Without the bonding jumper, the neutral is free to float, and though you may still see 120V phase-to-neutral on all phases, you might see one phase at 200V and another at 50V when you're expecting all to still be roughly 120V from ground. This tends to really mess with some equipment, especially surge supressors which are looking at voltage-to-ground and will discharge on the high leg.
The same is true of oil-filled transformers, as well; whenever there's a Y-secondary, you need to ground X0*.
* caveot for some generator/ATS setups, but that's getting a bit off track.
