The title says it all -- why are spell vamp and life steal separate entities? Would it not be easier to manage and simpler to understand if it was all changed to just be:
Life Steal: Heals your champion by a percentage of any damage it deals. Area-of-effect and damage-over-time abilities heal at 1/3 effectiveness.
Deep Terror Nami3 votesSimply put; Any champion that can benefit from both will break the system.
FantasySniper1 votesSuch as Jax, Kayle, TF, Akali, Lee Sin, basically any anti-carry that's a bruiser or assassin.
Lumus Avatar1 votesThen you would have too much utility on a single item. one heals using auto-attacks, the other heals off of abilities. Ability heavy champs choose SV, AA heavy champs choose LS. Hybrids have the option to build both (see: Hextech Gunblade)
Renzol1 votesHow would this work with champions that have abilities that trigger on-hit effects? Would it double it? I'm thinking of things like Ezreal's Q and GP's Q.
Drunk Old Man1 votesBecause it enforces certain itemization paths. Having 
apply to things like 
ult or 
ult would be.. bad. It would heal them for an insane amount.
RoyceRollin1 votesLife Steal: Heals your champion by a percentage of any damage it deals. Area-of-effect, ranges AA's and damage-over-time abilities heal at 1/3 effectiveness.
ftfy
Zhugan1 votesDear sweet jesus god no. The list of champions that would break is relatively small, but the champions on the list that would break would be absolutely terrifying. (I'm looking at you Jax/Akali/Nidalee.)
67chrome1 votesWell - as it stands, itemization in LoL does a pretty good job at seperating certain attributes. Rather than being able to get every stat avalible regardless of the champion or roll you play, certain atributes are more perks and rewards to sticking to a certain play style. Which, in turn, makes different play styles actually feel pretty different. Life steal is one of the big perks for auto-attacking champions.
As for combining life steal and spell vampirisim into one stat - you run into serious problems balance wise by pushing the 2 stats together. Rather than being an auto-attacker stat due to only working with auto-attacks, you turn it into an everyone stat.
For one - armor and magic resistance technically enhance all healing revived, life steal included. If you have 50% damage reduction and restore 10 HP, your opposition needs to hit you with something that does 20 damage to negate it.
Secondly - the tankier a champion gets, the more their damage is split. Tanks like Alistar are pretty much 50/50 with physical and magical damage (in part because getting minimal AP means base damage on auto-attacks still matters, but he's still got that 90 AD steroid). Tanky DPS like Mundo and Shyvana are pretty much 50/50, and Shyvana with Kingblade is pretty durable. Oonce you get to lighter fighters like Riven and Jax, a decent chunk of their respective kits can't use life steal either. You have to go all the way to pure squish Fiora and Tryndamere for life steal to benefit most of their damage.
With pure squish, you're still weak to burst damage and focus fire. Life steal requires auto-attacking to do anything at all, and regeneration is useless if you get one-shot.
Now imagine Mundo with a split life steal/spell vamp. His burning agony will keep inflicting damage while he's stunned, and you can't nuke him from 100% to 0% unless you farm someone like Nasus, Veigar, or Thresh for a week straight. Or just imagine other bruisers suddeny having Mundo's ultimate for free. Nasus, Renekton, Shyvana, Garen - healing off that AoE skill damage 24/7, along with the big hits from auto-attacks.
Combining both life steal and spell vamp would be life combining armor and magic resistance into the same stat. You'd just make it considerably more powerful.
Also, there's the drain-tank problem. Where tanks could build it (especially tanky DPS bruisers) and simply play tank'n'spank, which is the dullest, most interactive fighting style ever.
