I can see people getting away without most addons, especially as Blizzard has improved the default UI over the years and made obsolete (or at least relegated to a niche) some previously essential one.
However:
For some classes/roles/activities, you are probably losing out on a lot of convenience and/or clarity of information that could make life easier.
For example - in the screenshot posted above with the paladin, holy power is attached to the player frame which is in the far corner of the screen. That's pretty essential information to be at such a distance from the center of action. Now you can move the player frame in the default UI to be closer to where your eyes would see it more readily, but you can also separate and make the holy power meter much more bold entirely with addons if you want just that info front and center, and don't care as much about the health and mana bars being further away (if you're not a tank or healer, you probably don't care as much about those.)
Maybe not the most poignant example of an improvement, as there are many other more important areas besides moving frames around that addons can improve.
This is very true. It's amazing how archaic a lot of current UIs are in the MMO space or how unintuitive and a deterrent they are.
I'm fairly certain that if we took a sample of people's UIs here (who have addons and such) that over 75% of them would have the unit frames located at the bottom center of the screen around their action bars.
The way I see it is this: you want your eyes to be focused in one area of the screen as much as possible, you don't want to have to move your eyes from one side of your screen to the other frequently (as would be the case of a Ret Pally looking at their Holy Power). Though you don't want a total overload of data in that area, you want as much and as good of quality of data as you can get in that area.
Here's a screenshot of my interface:
http://i.imgur.com/lD8yW.jpg
The blue frame is the extent of what I probably have to look at at some point during a fight to look at things like DoT timers (I could move this, but I it would be clunky to move elsewhere), or the damage/threat/healing meter and Grid. But I can also clearly see any environmental aspects, adds, etc.
The green frame though is where I am most of the time. I can see what I am targeting, any power aura popups, my cooldowns, my action bars (consolidated to a single area), my DoT duration (on single target fights), out going damage, power gains, incoming heals, etc. So while I can see a lot of information, it's not inundating me.
Anyway, that's my little rant on UI design.
I believe all of the HoPo abilities light up when you have points and get gold borders when you are at 3 points, it might not be the most efficient way to play, but you don't really need to keep track of your points at all since the default UI will remind you.
I haven't tried playing with the default UI in some time, do they allow you to move the action bars around the screen and adjust their size now? I have also seen some people with their unit frames moved around, but didn't know if that was now possible with the default UI or if they were using an addon for that.
Yes, the abilities do light up but you need to mindful of how much you have for upcoming GCDs.
To my knowledge, action bars are still locked to where they always have been so if you've seen bars moved around they are most likely using an addon.