Twitch goes yomumu because he is an assassin ADC, so it only makes sense that he would buy an assassin item.
Draven goes duskblade because he is a burst ADC, and duskblade is a burst item.
MF goes Yomumu/BC because she needs armor pen for her ult...fucksake, BC was a staple on MF in season 3, and now people are suggesting BC to have half stacks on ranged champs? Aka how to break MF for no reason.
The only one who doesn't make sense is Lucian and he's getting nerfed, so chill please, those guys don't buy standard ADC items because they are not standard ADCs.
I do think IE-based ADCs could get an itemization fix, but this is another issue.