This is literally going to be a nightmare for players. Does it have advantages? Yeah, it does, but the disadvantages pretty much (in my opinion) outweigh the advantages.
You're going to have idiots with open mic on ALL THE TIME. This means hearing their crappy music, hearing their mom scream at them, their extremely heavy breathing, and just general background noise. You're going to have different personalities, and sometimes different languages and dialects, shoved into a voice chat together and who knows how it's gonna go. The only way to solve this would be to have ONLY push to talk, but even then people would complain about having to push an additional button to talk and that it would interfere with gameplay.
If you choose to opt out of voice chat, you're going to get flamed. If you team goes to make a play and they don't communicate with you, they're going to likely die and then blame you for not having voice chat enabled. This is going to be very, very crappy for people who are stuck solo with a four man queue.
If you're a girl, forget it. You're never going to use voice chat because people are going to harass you and be rude and sexual for the sole reason of you just being a female. If you're a feminine sounding dude, you're going to get called all sorts of slurs and very unwanted names.
Toxicity in general is going to be high, and if Riot isn't prepared to keep and review upwards of thousands of voice logs a day then this doesn't need to be a thing. If someone is being toxic in voice chat and you report them, someone is going to have to listen to it and determine if it's a positive report or not. If they're not going to implement a punish system for this, it's going to be bad and they're going to lose more players than they already have.
As for communication, Riot could implement better ways in game that things could be communicated. For example, a ward ping has been asked for multiple times. It's a real pain as a support player to ping, type warded, and go on all while still in lane trying to dodge shots. That goes for anyone, really. Knowing when someone is coming in/ganking/whatever is as simple as a ping or two on the map; surprisingly most people have decent enough map awareness that they're going to notice/hear a very loud unordinary sound.
Sorry if this all sounds really negative, but it is a legitimate concern. And yes, I'm aware that it's optional, but for a large chunk of the community this is going to be underutilized. Either people are just going to use it for their party and their party alone, or people are going to abuse it to no end until Riot actually realizes that it's a problem.
