Consider this scenario:
Life Orb Keldeo does 67.58-79.86% to 0/0 Haxorus, let's say Haxorus took SR damage and had to outrage against a hippodown in sand at +2. Haxorus takes 12.5+10+6.25+6.25%, ie he takes 35% Just to ko a hippodown, giving keldeo a sure chance to ohko it.
If Haxorus didn't take so much damage, Keldeo wouldn't have as much of a chance to ohko and more often then not an opponent would have to risk a pokemon both fast enough and strong enough to revenge haxorus, if haxorus has +1 speed this most often means a scarfed dragon/ice attack or a scizor pops up to revenge, hence FORCING the opponent to play a way you like (you can then set up/trap that pokemon)
heres an example of what I am talking about, the gem allowed hax man to ko a landorus-t that was bulky enough to take ice shard from weavile easily and then it survived a scarfed brave bird from staraptor with 3%... not possible for both to happen with a life orb! http://pokemonshowdown.com/replay/ou6638962
Staraptor and Weavile are too uncommmon. keldeo KOs haxorus with hidden power ice. If it doesn't have that, it won't switch. Worse, any steel could switch in not fearing haxorus less if it had a LO. Your scenarios are too isolated and would rarely if ever happen.