What's the point of Orianna's passive?

I was talking to people the other day about whether or not the new Nashor's was a good idea on Orianna. One thing just about everyone said to me was that the item and her passive are both traps and that Orianna should always be casting and never AA. These were people in gold, plat, and diamond too. One specific reason for that is they said that Ori can cast while moving and as such, you should never stop moving to AA. So I can't help but wonder, why does Orianna have a passive which tells people to AA yet better players are telling me that autoattacking is the worst thing you can be doing with Ori. It just doesn't match up. They also told me nashor's teemo does more damage with AAs than Ori does. What's equally baffling to me is I got a pentakill once using a nashor's build that focused on mostly AAing and making use of that passive.
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