Using voice communication is key in strategic games. I can't imagine games like cs:go, rainbow six siege, overwatch, fortnite etc played without a mic. I want to communicate with my mates, calm them down, cheer them up and convince to not give up or talk about our strategy, give useful tips or receive them. I understand that there will be a lot of people who will flame through the mic, but for them there is a mute and report button. It works in cs:go, dota2, overwatch and countless other games and I don't see a reason why it wouldn't in League. Premades won't have such a big advantage over solo players as well.
I think it would benefit League as a game. It's pretty odd seeing a team game without voice comms nowadays, every single one of them have it. Imo, it is lame and I'm looking forward to Riot looking into this issue. :(
And I think that people tend to be less toxic in voice chat. Idk, I just think it would be amazing to talk with your mates.