So League voice is good initiative from Riot(although late) but only keeping it enabled for the friends and other in lobby/ group and not whole team defeats the purpose.
So if we play 3 man premade and other 2 random strangers from Match making, we 3 people know how others play and generally are at sync what is about to happen in fights/ganks/ push. The more important part is how well the other 2 members understand whats about to happen.
Ofcourse, feature to mute also should be made available since voice flaming is more infuriating than text and there would be no evidence over it (may be unless if we report and the voice data is recorded and saved OR if we mute the teammate, all voice data 5 mins from that point is saved for review)
I suppose that would have a lot more in the strategic gameplay than current games on pings and text. And anyways trolls will be trolls xD