Consider this.
Without ground immunities the only types that resist ground are grass and bug. They both have two shared weaknesses, fire and flying.
Now consider this, all 3 of fairy's resists are weak to ground. 3 of the 4 steel resists are weak to ground and the other is neutral. 2 of the 3 normal resists are weak to ground, the other neutral.
Now add to it these. Under gravity nearly every move is 100% accurate, letting you throw around hypothetically fully accurate blizzards and hurricanes without weather at the same time. Most defensive Pokemon that are immune to ground + a good ground coverage type would often take SE damage from ground without the immunity. See Skarmory, Bronzong, Talonflame, Zapdos etc. Also, flying types, which take extra stealth rock damage but are immune to spikes, now take damage from both.
If there were an auto or perma gravity Offence would be super good.
