Just saying, turtling has won me a few games recently. Firstly, they destroyed us in the first twenty minutes, we pulled the game to an hour and a half and two of their people rage quit cause of it. Second time we turtled with a nasus and he was able to virtually one shot everyone by 60 minutes ^^. But they were all stupid and make horrible mistakes.
The first mistake was pushing our inhibitors every time they came up. That just allowed us to free farm inside our base as well as not providing them any advantage. We wouldnt even contest their attempts to take our inhibs cause it was too good for us.
Secondly, they waited too long to make a suicide run at our tower, if you do it early in the turtling you can usually take the tower with little effect to you, you ill be able to respawn before they can push more than one tower, then can usually kill them while they are out of position the the lanes/jungle. If you wait till 60 minutes into the game, of course they are going to take themselves all the way to your nexus when they ace you!