Everyone seems to think that Sightstone is a default, required Support item. It's not a perfect purchase every game. When I get autofilled into Support and forget to ward anyway, it's useless to waste the gold. When I'm playing in low MMR and no one looks at the map anyway, it's a waste of gold. If I'm playing in high MMR, using information given by the map alone is fine without wards (i.e. if the enemy mid is MIA, you don't need wards to know you should back the fuck up)
Not to mention that a trinket gives almost as many wards, and it doesn't take up an item slot. Sightstone is more of a fringe, occasional purchase if you ask me.