VoIP is very useful in the game for coordination. I have Mumble running off of a Raspberry Pi for my free company (and it's been regular content so far, no raids yet.) Most of us are also on PS4 so we don't really have a free hand to readily text chat. Voice is the most efficient way to communicate, but as you note - it doesn't work that way for everyone. It's still important to accomodate the rest of your team based upon individual needs.