The only thing I've ever seen this feature do is create a toxic environment. On the slim occasion, you'll see the 11 and 0 constructive Darius say, "Garen, don't build Tritinity Force. Build Black Cleaver since you're behind." Admittedly, this is me at times, since I don't want the opposing laner to quit their champion just because of a bad matchup or they don't know the champion they're up against.
Aside from the every 100th game this happens, the only point of /all chat is to tilt your teammates. I recently played a game as Ornn, going 7 and 0 in lane (something on those lines) and watching our friendly Soraka flame our Sion mid in /all chat for "not knowing how to play the game." Yes, this would have happened either even if /all chat didn't exist, but this is just one of MANY situations someone would start typing in it.
I kept telling the Soraka, "Get out of all chat. I'm a fed Ornn, I deal massive damage mid game, and can tank the entire's team damage if we close this out fast enough." Our ADC and her threw the rest of the game despite me carrying 3 teamfights.
Again, please remove /all chat. Thoughts?