Yellow Trinkets have a lower duration and are far more limited in their placement.
Tracker's Knife and Sightstone are both great for securing large amounts of vision in a specific area immediately (i.e. ward their bottom side jungle when pushing the inner bot tower) because you can drop 2-3 wards down at any moment.
While vision isn't the sole responsibility of the support, it is one of their core tasks. The support refusing to purchase a Sightstone is just plain inefficient; it is extremely gold efficient, protects the ADC (and occasionally the mid-laner) from ganks early-game and lets you control vision in the bushes bottom-lane.
If you don't buy it, you are just putting your team at an immediate and needless disadvantage.
No matter what role I play, I tend to spam wards around the map. I love having vision since it provides invaluable information and creates opportunities.
- When I'm jungle, I will constantly drop wards in the enemy jungle to track their movements.
- When I'm mid-lane, I'll constantly drop them in mid-lane (to see when my opponent backs or roams) or in common gank paths.
- When I'm support, I rush Sightstone (often into an Eye item) and light up the map everywhere I go.
Vision is just that bloody important.