For example.
Boss does an aoe every 30 seconds, you're told to move out of it. Every two mins casts a big attack that can be interrupted, this appears on a bar, you can interrupt.
Now what about that is hard ?
You repeat your main attack, cast rotation, shot rotation ect ect
You move when the bar gets to almost zero, and you cast interrupt when the other bar is on screen.
Explain, how that is easier without addons ?
Addon's make the game more accessible and easier, I've not said there a bad thing, in fact I have a huge list of them that I use. I just am of the opinion, that raiders or any player that uses them, is making the game easier, it's not a "must" to raid with a addon, it however makes it easier. You could get 25 people to raid without addon's, but it would be harder then with addons, they'd have to think and react much differently compared to having addons.