Most of the discusion isn't so much having it or not but when they get it. Everyone demands supports buy it asafp but that isn't always right or useful. Personally I don't buy one until I can afford the full item. Odds are if we are so far behind that I can barely afford a SS the vision isn't going to save us (from the most likely repeated 4 man bot ganks). And if I'm so far ahead that we are hyper crushing a lane why would I need it? At that point it most likely requires the whole enmy team to kill you and if you can't see all missing on a mini map then you have bigger problems than a SS.
I will get a SS (or occasionally a frost queen because the ghosts are actually really friggen Good at vision control as well if used right) in all my games. The only thing that would change that would be a massive buff to FoTM.
P.S. I don't buy coin because it just doesn't seem to help out that much so oasis /coin are a never buy for me.