I've not yet seen any video that would answer this, so thought I'd muse aloud here, in case someone HAD heard one way or another...
In Star Wars: The Old Republic, when you were engaged in a conversation with an NPC, you were invulnerable to all harm... I'm reasonably sure you could not ~initiate~ a conversation if you were already in combat, so it could not be used to 'escape' (and in any event once the conversation ended the foe would presumably still be there).
In every other MMO I've ever played, though, when you're talking to an NPC it's entirely possible to be attacked. Granted, most NPCs are located in relatively safe areas but with the ESO pledge to encourage exploration and acquiring quests through non-standard means, I wonder what would happen if I found some wounded soldier and started talking to him only to get ambushed by a patrolling foe.
Aside from the pain of taking damage before you realize you're under attack, and the lost time to back out of a conversation (assuming you don't automatically get ejected), the main thing that worries me is that the conversation will be 'lost'... that is, when you go back to talk to the NPC after defeating the interrupting foe, you find that the NPC just says, "You heard me, go do that thing I told you to do"...
The journal may have the entire transcript, which would be nice, but if it just has the keynote points, and the discussion is not repeatable, it's LOST. All because some skeever stumbled across my conversation. Or even worse, someone ran a train of frostbite spiders across me.
This may be entirely moot if all dialog is repeatable and if you're dropped out of conversations immediately upon attack, etc. so that you never lose any dialog options and get to experience everything properly.