Not Updated For Current Season
You are stupid - no one would ever ADC rammus
O.K. I want to be very clear. I'm not advocating this for a normal team comp. I'm not even advocating you do it often. What I am saying is that if you know how to last hit with Rammus, and see a 4 or even 3 AD team, he works. And with so many people playing Garen or Darius top, Yasou or Zed mid, and taking Yi or Xin Zhao jungle, and they are guranteed to have a "real" ADC - that this Rammus set up and build will shred that team composition.
Pairing with Support
Rammus doesn't pair well with a tank support like Alistar or Braum. Braum keeps damage off an ADC, and Rammus doesn't need that. A good stun or snare with high AP does best - Lux or Morgana being ideal, but you COULD go with Taric, and then you will be able to use both ults to shred through any armor.
Late game low damage
OK - BUT ADC RAMMUS doesn't do enough damage. Well, he does, and more importantly, he doesn't die, and it hurts everyone to attack him. That being said, if you have a jungler that is low damage, and a midlaner that is tanky, and a top laner that is made for sustain, you might have to go with something else like a ravenous hydra to get damage done during team fights.