Here's what I don't get. If the group requires or benefits from the raid leader's vocal call-outs so much, then the raid leader must have information that the other group members don't have. Otherwise things wouldn't need to be called out, right? However, the static leader isn't any more experienced with the instance unless they go practice with another group when content is out. This isn't a common practice I assume.
So essentially someone with equal experience is calling out things that everyone has seen in the videos and read about beforehand. What's the point? Coordinating cooldowns can be done with 2 macros: "Swiftcast on cooldown <se.#>" and "Stunned/interrupted <se.#>". You can also macro a message to some support/emergency skills. Coordinating positions should be done beforehand because they are mechanic related and mechanics are known beforehand. I can't think of anything that a good player would need called out in voice chat specifically.
My experience with it was that people chatted about random things and play bad music, strats which could have been compressed to a single macro were told over a longer time period than it took me to read a guide, the raid leader would either call out things I already knew or bark at people depending on her mood, kids would scream in the background and there's always one mouth-breather or keyboard-basher who doesn't use push-to-talk. Not to mention the drama of some hearing the voice of a real girl and falling in love... or even worse the people who take flirting too far mid raid. <_< And if a raid team enforces voice comm it's a choice between not raiding and putting up with mandatory chit chat for 2 hours when you're supposed to be unwinding from the day. Teamspeak can be fun too, as long as it's moderated, the people are decent and it's not mandatory for raiding or for knowing what the guild is up to.
I read about an all-deaf guild in WoW (2013) who tackled some of the hardest content in the game at that time, so it's possible to play at a pretty high level without voice communication for sure. TS is fun for socializing and useful for the crowd capable of making the strategy videos before any have been released. Otherwise I wouldn't say it's required.