We all know wards are very important & useful, but one thing I would like to know is, why is it the supports responsibility to buy sightstone & ward?
Shouldn't everyone buy & ward or at least the jungler since they're roaming?
I main Soraka 
by the way.
You are completely right. The reason is because...elo? I see professional games where everyone wards, etc, but the lower elo you get the more and more it becomes a support-only responsibility. I think the trinkets were suppose to mitigate this problem, by reducing the cost of other players warding, but it was only a partial success, due to player skill and 'carry' types being shortsighted in low elo play.
News flash: it's already commonplace for the jungler to buy a Sightstone. The support still buys one regardless, double Sightstone is the new standard.
everyone should ward
however, it is efficient for the team to have a dedicated warder with sightstone
also the support is often the shotcaller (like Yellowstar of Fnatic) so vision control is part of determining where the team will soon contest (dragon, baron, part of a jungle etc)
The support buys a sight stone because they're not expected to deal damage. They also need wards more so than any other lane because they're in a long lane with a low hp ADC.
We all know wards are very important & useful, but one thing I would like to know is, why is it the supports responsibility to buy sightstone & ward? Shouldn't everyone buy & ward or at least the jungler since they're roaming? I main Soraka
Not sure how long you've been playing but Sightstone and ward cap ,was honestly designed to alleviate the amount of gold Supports used to spend plopping wards down lighting up the map like a Xmas tree. (I miss those days, who ever warded more normally won) Back then still though it was everyone's job to ward, but the beauty of the support role is that they don't need items to fully function so it's best they invest in vision control.
but, having in mind sups cant 1v1, and to efficiently ward you need to roam, it would be more efficient if a champ with good mobility (like sups usually are not) or champs able to 1v1 would do it
If I am playing Top, Mid or Support, I will usually purchase a Vision Ward at the beginning of the game, and replace it if possible. I hate the idea of blowing money on the Sight Ward that will eventually die, but Vladimir is the only non-Support I've been able to tack a Sightstone on without much problems. I understand that Lee Sin also utilizes Sightstone regardless of lane but I don't play him.
Funny thing is, I notice my teams in normals take warding seriously when a Shaco or Evelynn is on the opposing team, but rarely other times.
