You make it sound like a LM army is weak without magic support... Not many armies get 4+ armour, T4 on their basic troops. Even less get to dish out 2 S4 attacks per model. The main thing that will drag down saurus units is combat characters and support like chariots and monsters. Lore of death aims to deal with both of them.
Well it is, and there is really no way around taking a Slann. Magic is our strongest trait as an army, and with magic being the balancer for armies without warmachines, we have to have it. Salamanders and the EOTG can only do so much, the Slann has to bring the magical buffs and artillery to plug our weaknesses.
Saurus will definitely chew through enemy Core easier with 'Soulblight' on, but they're still painfully slow (srsly, great weapons are just about the only thing they beat), and as you said there are plenty of things they can't handle without support.
Let me put it this way. In a list with a single Slann, I'd be hesitant as taking Death for his lore, as he's the only Wizard not forced to take Heavens in your army. In a double Slann list, Death has great synergy with almost every other lore, but especially Shadow and Light. So, with another Slann being the BSB+General+buff caster, you can head off with the Death Slann and his retinue to start killing stuff.
The biggest problems I have with Death are twofold.
Firstly, the short-range hexes and sniper spells are great, but they require you to move into range (to snipe the BSB etc). Lizardmen do want to close against certain armies (warmachine spam armies hit you from either distance, so the sooner you hit combat the better), but being forced into it is a bit dangerous, as it can hand your opponent the initiative (ie he begins to dictate where and when you can hit his battleline).
Secondly, 'Purple Sun'. If you miscast, or roll misfire on the scatter die, you can basically kiss the Slann and TG goodbye. Also, technically, it kills a couple of TG from the first rank when created (vortex has to start with the Wizard then flies away). Seeing as its the nuke of the Lore, and the ultimate (and the most feared nuke in the game), I'd rather take another Lore with a less suicidal ultimate.