I think your friends mean you shouldnt be committed to autoattacking. An ADC can get away with autoattacking and taking some damage in return, since they can lifesteal it back up. You are almost just as squishy but without the sustain. Buying a Nashors on Ori makes you committed to autoattacking or else its just a gold dump, and that means youre spells do less damage.
Especially considering the # of items you can buy. What would you get rid of to make room for the Nashors? Ludens with the passive which does more AoE than the Nashors? The Zhonyas for the clutch saves? The Deathcap with the multiplier? You kinda need Boots/Void/Morello. And even Rylais gives more AP, gives health, and gives you more kiting potential. IMO, even Lich Bane would be better, just for the extra burst.
And Teemo does 50+30% on hit from his E. Orianna only does 50+15% up to 60+18%. And Teemo also has better base stats in terms of AD and attack speed, plus you also get the benefit of his poison DoT for every auto you do hit. And that doesnt include his passives attack speed. Buying Nashors on him would be better than Ori, but even then i feel like its a gold dump. Ive done a lot better with Lich bane on Teemo, since he gets a free auto reset with his Q - which is another thing: you cant do as quick of an auto-spell-auto combo with Ori, since the drawback of being able to cast while moving is that it doesnt cancel animations.
EDIT - about the UoL using runeglaive/Nashors Ori. I think it was a decision against the teamcomp. Enemy team had Gragas, Lucian, and Ahri. I saw 4 ults catch one person (support twice, ahri once after jarvan locked her down, ahri once in lane) and 2 people once (gangplank and rek sai). I think he knew going into the match that against such a mobile team, he wasnt going to get many hugeults off. On top of that, GMB didnt have anything to threaten her with a gangplank top and gragas support. The only champ that could threaten her was Ahri, which was why Maokai was on peel duty most of the game instead of diving. In this case, UoL got away with the runeglaive/nashors Ori because the enemy team was squishy with no backline threat. However, im willing to bet that if he got 3-4 person ults off, he would been able to delete people anyway