Sightstone and Why its not Mandatory
(This post refers to solo/dynamic que and not high end organized team play where the game completely changes)
Sightstone is a fantastic Item and definitely has its spot in the game. BUT along with all items in the game it is a SITUATIONAL item.
Consider this list of ward spots:
Core Wards: 2 main ward spots in the river 1 Ward Baron (replaceable with skitter) 1 Ward Dragon (replaceable with skitter) 2 Wards for mid lane jungle entry 1 for each side. (usually only need 1 out at a time depending on side of map) 1 Enemy Blue buff intersection ward (only when pushing)
Secondary Wards 1 Split push top tri bush ward 1 Split push bottom golems bush ward 1 dragon tri bush ward 1 baron tri bush ward 1 dragon/red buff bush 1 baron/red buff bush
So now lets do some math. Out of the core wards you only need 6 of them ever at a time assuming all skitters are unavailable. A normal team comp will have 1 player with teleport so they will keep the normal warding totem. 1 person with a sweeper (jungle or support) and 2 with farsight alteration and the last person with a farsight or regular totem. So this equals 4 wards at least. Jungle and Support purchase a pink ward each. We are now up to 6 wards covering all core wards. Top split pushes and purchases a pink ward to cover his core ward. We now have 6 core wards taken care of with 1 extra ward for him to cover vision while split pushing. Normal Totems hold an extra ward over time which fills in the blanks when taking baron or dragon. So we are now up to 9 wards 3 of which are pink wards and we have covered all but 3 situational wards at any given time. Fight Sight alteration wards stick around forever so they can easily make up for those 3 wards.
So in an AVERAGE game there is 0 need for a sight stone as to light up 90% of the map you only need 12 wards. Also remember minions, towers and other players also give vision. With map awareness you can simply see someone walk in a direction and know where they will end up you don't need to see their ever step to know where they are.
No sight stone means more money to get other items which help more during a team fight.
But what about those games where the enemy has a Teemo and shrooms are everywhere so everyone is buying a sweeper? Then you get a sight stone. Both jungle and support get a sight stone.
Some of you might argue there are more warding spots and you are correct there are a few more warding spots but most of these were left out due to another ward would give better map awareness making that ward very low priority and not often used.
So TL DR stop thinking sight stone is mandatory because its not. So learn how to simply ward better and have better map awareness.

into
problem solved
is the best item you can buy. Everything ELSE is situational.