I feel that VOIPs are optional for a lot of things, but I think that the SE design team decided to make it essentially mandatory for high end raiding. For most raids it can be bypassed entirely as the fights are extremely scripted, but my static actually had to replace our non-voip member once we got to AS3. Having to coordinate stuns/silence on 4 mobs that spawned in random spots with the party members also randomly positioned when that happened simply could not be typed out on the fly, and we weren't going to give 1 person zero added responsibilities for the encounter just because they didn't want to voip. My group now does have 1 person who doesn't voip, but he lives in a house with another member, and the both of them are also on PS4 chat, so orders can be relayed in worst case scenarios.Because sometime things go wrong in progression or even farm fights, and telling the BLM to put Apoc on someone or Lethergy the Liquid gaol can be the difference between a wipe and a clear.

TL : DR I think the VOIP is optional, but endgame is designed with it in mind