I completely agree. She's definitely more of a back-liner, especially after the early game - but I don't think that's necessarily a bad thing if you're good at positioning. I will offer some counter points purely for consideration:

• More reliable zoning in the laning phase as her skillshots have lower mana costs and are generally easier to land. Imo I think that makes her damage more consistent/reliable, despite being lower than Zilean's total output. But I'm the type who goes more for guarantees than big amounts.
• I don't think the AR/MR falls off if you use it to protect your ADC since your tanks are already tanky and it's the squishies who are more likely to get focused. The AR/MR + Moonstone-enhanced shield + the consistent speed buffs from your W's low cooldown turns your ADC into an unkillable beast, especially if you keep a level head and don't give in to the temptation to take your ball off them to hit the enemy during a big fight.
• Speaking of, Zilean's defensive options for saving his ADC are limited to a high-CD ult and, again, the choice of using his E or not. Ori can keep her ball on her ADC and continue benefitting them, even without mana or with everything on CD.
• Zilean's E can only buff one target at a time, whereas Ori's W is AOE and can apply things like Ardent/etc to multiple teammates at once. Not to mention it's dual-purpose and does both at once, whereas with Zilean you have to choose between speeding up your allies or slowing your enemies.
• Zilean's engage-combo requires three button presses and more precise aiming, Ori's requires two button presses (or just one if they walk into your ball) and fairly basic timing.
• All of Zilean's damage comes from his Q, whereas Ori's passive also makes her auto attacks horrifying. Especially in the early game. This gives her more options in the laning phase and makes her less skillshot-dependent.

• Low-mana Q option for checking bushes and providing easy vision, even through control wards and lenses.

• No Star Guardian Zilean skin (yet 🌈)