The point is, oracles in it's current form completely shuts down guys like Twitch and Evelynn, it completely destroys the plans for those who like to ward the map, it turns Teemo's ult into extra gold, and so on. 400 gold to make a champion lose it's edge. 400 gold. Thats way too cheap. Only thing which make Evelynn and Twitch stand out, is their ability to stay stealthed. Oracles fuck this up big time, and the nerf to it will make it actually plausible to play as these champs. No longer 400 gold to turn the game into 4 vs 5.
And if someone touch - especially if they have CC - is in the control of the oracle, your Eve/Twitch can't go near them without getting torn to pieces. Shaco's ganks become as effective as ganks made by lvl 3 Soraka. If the enemy bot support gets oracles, they now have absolutely complete control over dragon and the whole bot river, unless your support also buys oracles.
Nerfing it to last only 2 minutes will be good. It can be bought to check all areas Dragon/Baron before attempting to kill it, since wards will most likely become more common if Oracles get nerfed. It can be bought if you know you need it in a teamfight with a stealther. When you could buy only one oracle every 7 minutes, people learn to ward & counter ward better. If the enemy Twitch/Shaco gets fed, you don't need to keep burning 400 gold over and over and over again to counter them: Forcing people to learn warding will mean that they won't be dependant on oracles when against stealth. Currently it is the only way to shut stealth champs down effectively, but it is doing that a bit too well.