User blog comment:YourPrivateNightmare/Hail the hourglass!/@comment-3466579-20130915093505

Please note that Zhonya's works on APCs also because they usually have burst damage unlike ADCs with auto attack based, constant damage. After a full combo Ahri doesn't have much to do so it's a nice idea to get Zhonya's shielded while under attack, to gain some seconds while your skills get back off cooldown. As an ADC you might as well keep on attacking, life stealing on your 900 damage criticals while keeping yourself out of danger. APCs and ADCs do different things in different ways, let Zhonya's Hourglass to mages.

Not to mention that Zhonya's active is in no way better that Guardian Angel's one. One prevents damage (you need to know when it's actually coming...), the other one gives you another chance for fighting after the damage has actually been dealt.

I'm in the middle of Nunu's ultimate? Nice, i'm going to activate my Zhonya's. I than take Syndra's ultimate in the face, but whatever. Now i'm in the middle of Nunu's ultimate? Unlucky me, i'm going to take a half hp bar damage. Oh crap, now that Syndra's using her ultimate on me, i'm dead. No way, i've got my Guardian Angel.