I think that on a basic level, a good user interface is crucial to an immersive MMO experience. I'm not talking about details like the size and exact positioning of your action bars here, but about the very way you interact with the world. It seems to me that there's always a balance to be struck between immersion and convenience: Pushing a button to instantly teleport from anywhere to anywhere else is certainly convenient, but it also means that there's no real sense of place. On the other hand, running around a beautiful landscape with no understanding of where to go or how to interact with anything will soon leave players feeling lost. The trick is in finding the sweet spot in the middle.
I've been thinking about all of this recently because of nameplates. You see, the default setting in World of Warcraft when I first started playing that was to show nameplates for friendly players and nothing else. It being my first MMORPG, I of course didn't know any different, but even once I became aware of my options, that setup still made sense to me. I get that characters having their name float above their head isn't in any way "realistic", but it enables the player to recognise familiar faces out in the wild, something that their character should reasonably be able to do in-universe as well, but that could be hard from a player point of view when all you see is a generic character model in the distance.
Eventually I learned that there could also be a benefit to turning on nameplates for friendly NPCs when doing the quest for master first aid in WoW, which is basically a slow whack-a-mole that requires you to click on a number of injured NPCs in the right order based on their names, but the actual character models all look the same. If you have to click on each one first to see what they are called, it's nigh-impossible to get right within the allocated time. If they all have their names floating above their heads at all times, it's trivial. So I got into the habit of turning that setting on for that specific quest and then immediately turning it off again.
I've been following the same model in SWTOR since the beginning, but just like Revan forced me to re-evaluate my view distance, bosses sometimes have a way of favouring one UI setting over another. We've been running veteran mode Gods from the Machine again recently, to teach some newer players the ropes, and the first phase of the Scyva fight involves dealing with a number of small adds with relatively little health, at least some of which need to die in the right place and at the right time. When I was first learning the fight myself I found that part to be a bit of a struggle, but then I noticed on someone else's kill video that they had enemy nameplates turned on, so I followed their example and voilà: instantly it became so much easier to keep track of all the little adds in the room and what health each one was on.
Back then I still stuck to my guns and turned enemy nameplates back off after the fight, but this time around I keep forgetting and it's been making me thoughtful. When tanking a flashpoint it's so much easier to keep track of all targets when they have their names and health floating over their heads, plus you can make much more decisive switches based on health levels.
Just cruising around the galaxy has felt very different too. Out in the wild, I can see even smaller mobs from miles away and it's much easier to circumvent them. I've long had a reputation for being oblivious to my surroundings and frequently pulling adds by accident, but now I'm wondering whether I've simply been made to feel like a buffoon because the people I've been playing with all had enemy name plates turned on at all times. It's not hard to avoid those!
As a result, I'm kind of torn. Part of me just wants to go back to the way it was already, to a landscape unmarred by floating names everywhere. It's so much more beautiful! However, the difference in convenience has also been very noticeable, and I fear that I may well end up missing it, especially in group content. Of course there's nothing stopping me from simply switching my settings around every so often, but still... it's given me food for thought.