First: this is not my personal opinion on whether I specifically should be building SS.
- Riot released a statement basically saying that, as a support,
Riot did not say supports buy sight-stone. They said they acknowledge supports are expected by non support players (not by riot) to purchase sight-stone, and they believe the choice of delaying or outright not purchasing the sight stone is a legitimate game-play choice.
You have taken their acknowledgement and blown it out of proportion and (falsely) you claim it means riot is officially saying supports must purchase sight stone or 2 week suspension.
- Support job is to provide vision and support for team
how do you know? That is not necessarily the optimal path. In fact, I am more and more convinced lately that marksmen should be getting the Sight-Stone, and also leave all the farm alone and let the Zyra or Brand gobble it all up while they scale off passive gold and provide vision and utility while they are weak.
If we look at typical end game stats for Brand and Zyra, we see a lot of damage output. More than your diana. Since it is your job, as a mid lane, to provide control of the map in the mid game, and you have more opportunity to roam and more capacity to 1v1 opponents if you are caught, it stands to reason to have the responsibility of team vision to be for the mid laner.
What I am trying to say is this. What we have now is a "meta" which has developed organically. It has developed in some part because it is "statistically" optimal, but also because of a lot of stagnation, it stays the same even when it is shown it should change. the stagnation is caused by selfish players trying to throw "un-fun" tasks in game like warding onto others, or are just fixated on their way of playing.
For instance for about a year and a half the "optimal" play style was to organize a lane swap and to 3v1 the enemy first tower, and from the other side of the map have the top laner accompany the jungler and steal half the junlge XP. That was the optimal competitive play - y did we not see this in soloQ? because players are too stagnant and need their "comfort zone" and cant deal with different strategies even when they are shown time and again to be outright better, when they require first coordination but more important than that self sacrifice.
- Our job is to do damage.
From looking at a small sample of Zyra supports. We examine the stats of its quite obvious that Zyra's job is to do damage. In fact since Zyra support is capable easily to out damage the mid laner, purchasing an item that provides neither damage not opportunity to deal damage is a waste, on her.
Actually, it really is a waste on her, considering she can out-damage the typical mid lane by a LOT if she is on equal farm.
Whereas a lane champion like Aurelian Sol can easily start off by rushing a Banshee's veil or some other tank item and focus on map control using a sight stone.
The specific function and responsibilities you have in game should be determined by the circumstances of the specific game, not just by what button you clicked when you queued. I understand you want your role to be "to jump on an ADC and kill him" - that is a fun responsibility whereas "to take a detour into the enemy blue and drop a ward on it" is less so, but the sight stone should be purchased by the champion most suitable for using it and least hindered from purchasing it.
I am really not certain that in today's meta that champion is always the support. If it is not, the play style that puts that burden on other players should be open for exploration, not shut down by "I dont want to get it and the support is not getting it, RIOT please fix this piece of shit support player with a ban hammer and also get the economy of the roles proper so that the zyra is incentivized to be the one getting it".
If you are a jungle, you are forced to take smite and forced to purchase a jungle item.
yes but you should learn your history before you talk about machete. Also, it upgrades into a ward item so why are all the junglers getting challenging smite?
The jungle item is designed to do two things: to keep jungler's PVP power low at the super early game (the jungle used to provide more XP and gold in the really early game, leading to junglers entering your lane in levels 2-3 and just 1v1ing you, and also at level 6 doing the same with some cheapo items), and to keep laners from poaching (junglers after level 9 would find most of their jungle taken by their team and would fall off really hard because of that).
Without machete we have two problems in the jungle: Lee-sin and similar junglers just fuck the lanes up early game with very little counterplay due to level / item advantage (and the fact that they have good early game), and carry type jungler being non-viable due to problem with getting items on time past 1 and a half items (because your mid takes your chickens, your bot takes your krugs and your top helps himself to the wolves and gromp) .
The whole "forced into smite" thing - its just something that accompanies the machete thing. Junglers actually need that "meta coercion" to keep the role from overlapping with the "non farming" role too much. Otherwise laners will actually get machete and start poaching.
When I play Diana, my job is to either provide sustained damage or to nuke squishies
I disagree. As a mid lane, your job is to provide control over the center of the map and give your jungler a fist to shove up the other team's behind. Whether this is best done with damage items, tank items or a sight-stone is up for debate.
So let me get back to the point of the post. Riot told this guy that supports should always be buying one of the sightstone variants and people absolutely flipped.
People flipped because of people like you not because riot said anything about sight stone. People don't like coercion from team mates and fellow players. When they hear that people like you have a problem understanding simple texts from riot and are going to go on with your attempts at securing riot's support in play-style coercion, they get mad at you guys.
because you are out of line.
who can speak about their experience with "must-buy" items in certain roles?
Lets say I am playing Support Trundle. Or you are playing Support Diana. What is the justification for having the sight stone on us? From a champion power fantasy aspect, there is no justification.
If we say our champions are supposed to kill shit, regardless of role, than to do our jobs as players of those champions we should be buying items that have good synergy with our champion's core identity. As trundle I want the most tankyness 
and the most AA damage 
- not some half baked not that tanky item that sets me with my limited income behind. Same with your support Diana.
This leaves "role" as the only justification for getting the sight stone on the support. The argument that "the support is the one expected to do it" is not good - because we want to win and to do the best thing for winning not the best thing for our team's happiness and satisfaction.
Therefore maybe sight stone is better on another champion? is it possible? if you can't prove that it is always in all cases better on the support, than the option of not getting it should be open for the support player to take.
and yes, if the support decides to go full damage and not ward, it maybe means the job of warding is Diana's.
or 
. Looking at all recommended guides, one of those two items is a must-buy. You never see Diana players complain. Our job is to do damage.
+ 
(adjusting stats of course).. make the item worth getting. That being said I main support and always buy sightstone... warding is part of your job as support.. do you really want your ADC wasting gold on SS and delaying a key item? ..
