Given how much trouble it is to click, or even just see, stuff like Summoning Bells with just nameplates being the obstruction (besides character models), I'm willing to bet that chat bubbles would end up being a clickable portion of said player... meaning total blocking of everything it covers. Yeah, some games do it just fine, but I wouldn't put it past SE on this matter to ignore the annoyance it would bring here. Even if just optional, the functionality of it would be very limited due to potential hindrance it can cause in and out of combat for those who select it.