It's not the supports job to be the only warder, every champ in every position should buy wards on every back --- *Claims Kokonoe Rin, Vita, Nagi, Evangeline, Hanyuu, Cao Cao (KM), Kaguya, and Flonne-sama*
Every role should be buying wards if they want to avoid being ganked. If the ADC has enough gold after buying his items after his B then he should buy one as well. Though I'm guilty of not buying enough wards midgame since almost every time I go place some wards my team decides to go start a team fight
if you dont mind dying when they realize you're not warding then dont buy them
if you dont want to die tho i suggest you buy them.
unless you think you can escape without them (like ezreal or something) then its probably a good idea to buy them yourself and stop acting like its never the ADC "job" to ward :D --- LoL summoner: Vejitables Bromaciaaaaaaaaaaaa!