I never claimed it was only the support's job to ward. I simply said that supports are often the ones who are encouraged to purchase sightstones. Though the only incentive is not very straight forward.
I thought i made it pretty clear that this is not so much about encouragement as it is association, which is why the actual gold return is so low. Most people associate ward coverage with a vague benefit of information, as opposed to the physical benefit, gold advantage, which that information provides. Basically the idea is that by giving a more immediate and identifiable apparent benefit, they will see success and be more likely to associate the success with the act of warding because they feel they are seeing the gold benefits of warding.
As far as people placing wards in base to get free gold... that was the point of the statement "Nothing significant, but maybe like 20 gold", that was an estimate, but remember this gold would only apply to sightstone items. So if i were to actually calculate the amount of gold which would be needed to make it insignificant...
A base sightstone costs 800 gold. A normal ward lasts 2.5 minutes. Max of 3 wards.
(These are done assuming all 3 are placed for gold gain and none of the 3 are being used for lane vision via trinket)
f(x) = (800/x)/3 * 2.5min = break even; where x is gold gained per expired ward
f(10) = ~67 miutes
f(15) = ~45 minutes
f(20) = ~35 minutes
This equation factors in the stat cost of the health on a sightstone, but i'll leave most of those calculations to you. HP value is 2.67/pt.
Equation: f(x) = ((800 - 150*2.67)/x)/3 * 2.5min = break even; where x is gold gained per expired ward
f(15) = 36.315