yes they have, i play with a few addons, but i can play just fine with 0, but it wouldn't be great. part of the problem is the blizzard ui is lacking a lot and addons fix the issues.
i use Bartender pretty much just because its easier to set / change keybinds than the stupid default menu and i can move the bars to the bottom right corner of my screen where they are out the way.
healbot, better raid frames, cba setting up macros to go with defualt frames, easy to set up, can't play my healer without them :\ probably my most important add on.
and thats really the only addons i use, i got custom cast bar ones, but i can easily do without that, thats just aesthetics.
i do think some players have too many addons, see people with like 60+ addons, just crazy! also i remember an addon from wotlk cata (not sure if its still around) but it was for ret paladins and it just flashed the ability you should use, thats the lamest addon ive ever seen.