Back on topic...
I agree, personally this is a feature I'd love to see implemented. It was a staple of classic 2D MMOs and online games such as Ultima Online and Graal and whilst I've never played GW2 extensively, it was a great addition to WoW. I never expected to like it but it was actually a really neat feature that felt like it added a bunch of extra life to hubs and gathering spots even if I didn't have any interest in what was being said.
However
I've always been under the impression that FFXIV's 'in world' text rendering was highly inefficient and surprisingly resource hungry, something that can quite clearly be demonstrated by toggling name plates in busy fates. Thus I think that whilst it'd be pretty straight forward to implement speech bubbles, I don't think it's quite that simple.
If I had to guess why we've not got them already... My first guess would be that it's simply a performance precaution for people on more basic hardware, adding it in could have the potential to nose dive people's performance even further beyond what we already see in busy areas whilst also potentially being open to abuse. My second guess is that it may even still be a hardcoded hangup from the almost comical UI limitations the old PS3's drastic lack of ram used to place upon this game. It's not been that long since SE were literally having to remove portions of the UI to add in party TP bars.