During the laning phase, you should always be getting a ward or two every time you back to prevent ganks and have good map control. Top lane should ward the river and possibly the tri bush depending on how hard he is pushing while the mid lane should have both top and bottom brushes warded though you should ward the top one if you only have one ward. The jungler should also be getting a ward or two at every back since he will be all over the map and should ward jungle entrances or for your lanes if they have run out of wards. The ADC is the only one who shouldn't need to buy wards every back since the support will usually have enough.
So while most people are good at warding their lanes during the laning phase, it is extremely frustrating to me that people other than the support stop buying wards once the laning phase is over. You should still try to buy a ward or two if you have the money to spare. Even when I play ADC I still buy wards occasionally.
With the Ruby Sightstone, I've seen a lot of supports simply stop buying wards once they get the sightstone. While it is a great item, 3 wards is simply not enough to get good ward coverage. When I play support, I prefer to have a stack of 5 wards in addition to the sightstone. This gives me 8 wards at once and I can easily ward half the map with it.
When I watch pro games, I always see that a majority of the team has at least one ward. Why can't people buy the ward? Wards pay for themselves by getting you kills and preventing deaths. They can win you games if you catch someone out of position or with that charm, bandage toss, ect.