The core is not to have emotional voices, but to train neural networks to emulate emotions (not just for voices). Humans are very emotional beings, and if you want to communicate with them effectively, you will need the emotional layer. Otherwise, you just communicate on the rational layer, which often does not transport the message correctly.
Think of humans as 20% rational and 80% emotional.
And I say that as a person who believed for a long time that I was 80% rational and just 20% emotional ;-)