Do you think it's worth it to stack Randuins for the 10% crit damage reduction vs teams with Yasuo and an ADC building heavy crit o
n champs like 
? You miss out on possibly another active but get to live a bit longer vs people with 
.
Or how about stacking 
to help boost 
damage?
U can get away with stacking if they don't have the same passive. Some items you have to watch for that.
I think there are more than enough viable items that there is no need to stack. With randuin's, you will lose the active and passive. With Sterakk's you will lose more than half of the item's value.
The only item worth stacking is 
they don't stack efficiently
