I personally think MMO's feel cheap without the option, it makes it more difficult to see who is talking (especially since name are auto hidden).
If ESO were to add chat bubbles, would you be fine if their was an option to hide them like the name plates?
Also chat bubbles make it easier for you to talk to the person next to you while zone chat is going crazy, but Darmonic just leave Zone chat.... No I am looking at zone chat for things perhaps, but if my friend wants to say something I can easily read the words above his character without having to scroll up and down the chat, which is actually pretty annoying since you have to open a menu to get a free mouse in the first place...
Also Roleplay is better with chat bubbles.
http://massively.joystiq.com/2013/08/02/tamriel-infinium-the-elder-scrolls-onlines-rationale-for-rolep/