Lmc is not really needed on a whammy or sampire,but very very nice to have.
One reason is,you spend a lot less mana on weapon specials and spells, so you can do a lot more with the mana you leech back.Especially in situations where you cant do a lot of damage for whatever reason and still want to be able to keep things like feint running.
Other reason is, after a mana drain or vamp, you are much faster back into business.
You also get away with a much smaller mana pool, with 40 lmc, 40 mana or a bit less is perfectly fine.
Just some numbers as an example. Using 40 lmc and full mana discount, first armor ignore or double strike is 12 mana, next is 24 mana. Very easy to keep up with that even with fast weapons that do less damage.
Now, with 0 lmc and full mana discount, ai or ds are 20 mana first and 40 mana next.Much harder to keep that going.
For weapon mods, mana leech is still needed as additional mana is always welcome and mobs will run out of mana also.Next comes hld, either from weapon or glasses.
HLA makes stuff easier, but for most mobs you will do fine without. Stam
leech is always needed except on weapons with 2s speed and 35 ssi ( 35 ssi tessen do rock against some tough stuff on a whammy).
Life leech is not very useful at all except for non boss stuff that you can double strike spam for at least 100 - 120+ combined damage every swing, it somehow starts to work very nice then and you dont need curse weapon.Works also very well on a whirlwind weapon if you got enough targets.
Against bosses,you just keep curse weapon running and life leech will add very little here.