I used the default UI for years before I started making my own interface late Wotlk. For me it's just a matter of taste, I really dislike the layout of the default ui but it's functional enough so frankly don't see a reason to bash people for using it.
I've been running a modified Elvui for 2+ years now but it was very difficult for me at first to get used to having my unitframes in the middle of the screen, I believe it took 3-4 months before I was used to it. Now I tend to center all of the important information I need.
in this world full of people there's only one
and if we only live once, I wanna live with you
Don't need any addons for pvp. Many rank 1 players use 100% default. Makes you more situationally aware.
I used default UI no mods. Well, gladius after I passed 2200 cuz someone told me it was useful. But only for arena
Default toolbar+DBM is really all you need to do proper raiding, everything else is just fluff for the sake of convenience.
Fod Sparta los wuth, ahrk okaaz gekenlok kruziik himdah, dinok fent kos rozol do daan wah jer do Samos. Ahrk haar do Heracles fent motaad, fah strunmah vonun fent yolein ko yol.
Back when I played I only used DBM, atlas loot, and WIM...I didn't see much need for anything else...and even then, half the time I would turn off DBM when I got used to a fight (usually a boss will say something before a specific spell or they use a set rotation/based on health when they do abilities so I never really worried too much..DBM gets annoying when you are raid leading in vent and constantly hear RUN AWAY LITTLE GIRL or those annoying dun dun dun sounds). But I have never seen any addons as necessary. I played the majority of my WoW career without any (I didn't really know enough about them at the time and just didn't wanna feel like I was cheating lol then I learned they weren't cheating they were just to make gameplay less clunky.) But I still think whenever you have people who complain the game is easy, I just wanna tell them to turn off their addons and play without them...the only reason the game is as easy as it is is because pretty much every healer, tank, dps uses the same 5-10 addons that makes playing their class easy as crap...I have high respect for the players who play their class well and don't use addons, simply because it is more of a challenge that way.
But I see the need for addons, since most of the playerbase probably doesn't do too well without...but I have never been a fan of addons that change the actual LOOK of your interface (as in character portraits/raid UI/spell UI/etc.) I prefer to just use the party/raid tools WoW gives me, and like I said DBM is really the only raiding addon anyone needs, but it doesn't have to be used ALL the time, it's just nice for when you are learning a fight and might need that little reminder that the boss is about to charge or flame breath you or whatever. I have tried using some other addons, like ones that tell me when my cooldowns, as well as other healers/tanks/dps cooldowns were up (I forgot the name of the addon now but it was fairly handy for raid leading at the time, my only issue was it was constantly popping up making it somewhat annoying and I only used it for about 3 weeks before getting rid of it).
I think that people here are just trying to justify using addons, and that's fine and dandy if they do. But they shouldn't be angry at those who don't need more than a few simple addons. My old guild master used to use just about every addon in the freaking game it seemed like...she was ALWAYS looking for another addon to try out...her son on the other hand honestly only used maybe 5 addons total (his computer would crash a lot so the less he had the better).
And as I said before, some addons are really useful and great. Other addons I think the game and most players can do without, and those are the addons I like to refer to as crutches. I think some addons genuinely do make the game a better experience...others are just, "I'm too lazy to pay any attention so I am just gonna get this to remind me when I need to use my finishing move" kinda crap. Everything has its place...but even if someone doesn't want to use the addons that make the game run smooth, it's their choice at the end of the day. If they can pull their own weight and not mess up without using the addons, then wonderful. If they happen to struggle though and get that, "I don't use addons" attitude, well that's just a shame. Like I said, I used DBM mostly to get used to fights, once I learned them I usually turned it off. That is an addon that makes it easier to learn a fight by walking you through the major steps. I think it's great for anyone who is struggling with raid fights to get it. And that goes for any addon that makes gameplay smoother, even if you just use them for a short amount of time to help with something you may be having issue with, it can still help in time.
But enough about that. I think I made my point.
That "awareness" you think you get is probably a placebo, especially if your raid leader expects you to know to know strict timers. It'll be within a bigger margin of error than having the exact same timers of your raid leader.
Again, personal preferences and fluffy placebos. Previously raiding with a bunch of stupids, timers were crucial when our raid leader couldn't call every dangerous spell out. Kinda helped that some other officers could agree on what each of us called out to ease the tension.
I see nothing wrong with the default UI either, especially now when it was updated with raid frames. Though I am missing the UI I used back in Molten Core days, however once Burning Crusade came it was never updated
I've played with the Default UI for about 7 years. Gave ElvUI and TukUI a try, but I just couldn't get used to them and I quickly noticed that I lost DPS because I pretty much lost track of all the things Im used to. And to be honest, I've seen a couple of people who are into HC raiding that use the default UI (even some healers).
It's a matter of preference, and as long as you like it, it can't possibly be a handicap