Its more for the mana return so she can stay in jungle longer before backing. +200 mana and 10% CDR, all stats she needs badly. The Spellblade passive is just bonus.
I prefer this over Cinderhulk because, while Cinderhulk does help with her clears (so does Runeglaive), it does nothing else for her. No AP. No mana. No CDR. Just HP. It does synergize with the HP on her other preferred items like Liandry's and Rylai's. Basically, if for whatever reason you need to go tankier (you get counter-jungled/ganked hard), then you can go Cinderhulk and still be pretty effective.
However, most folks forget that her human form W deals AoE magic damage with an 80% AP ratio. It does tons of damage. And people forget that her Q does %HP damage that starts at 8% and scales up with AP at 3% per 100AP. Her basic attacks in spider form are 30% AP on-hit magic damage, and heal her for 10% AP. Finally, her spiderlings' damage also scale with 15%AP. So yeah, the ratios may be poor, but they all scale with AP. So building AP adds up rapidly. And since she does all magic damage, building magic pen is essential. So sorc boots and Haunting Guise are core.
A lot of people still don't rush the Liandry's. But since it got buffed, its really good on her. 80AP and the DoT is real folks. If you full rotate from human form on a stunned target, they will take 4% current HP per second for as long as they are stunned (it checks every .5 seconds iirc) on top of her Q's %HP shred.
This is why Rylai's is also so good on her. All of her abilities are either AoE or single target. So she gets the full 40% slow, which procs Liandry's double DoT damage while keeping them in melee range. And more HP. And more AP.
And because the slows and DoT are on-spell-hit abilities, CDR is important to proc the passives more frequently.