Late game, it's not odd to have more than 1 stack of wards. Catching someone alone late game or having a vision advantage of the enemy team for a team fight could drastically turn the tide of the game.
For example If you ward an entire side of a map and then see the other team's AD carry overextend even just a bit, a 2-3 man gank would be enough to kill their AD and then immediately grab blue buff/dragon or baron.
Warding buffs, dragon, and baron is only good if your team can contest it. If you only place wards at those 3 places, what good will it be if 2 people on your team get caught in the river bush and die?
Multiple good ward placements define higher level play.