Whether certain information necessary to properly play the game is part of the base user interface or via a third party add-on isn’t really the issue for most (imo).
The issue most have with add-ons is that it requires a lot of extra knowledge to optimally and/or properly configure the various add-ons, knowledge that goes well beyond the scope of the game.
For example, while I’m sure most on this forum will disagree, figuring out how to configure weak auras to provide audible or visual queues for countless abilities is complex for the average gamer, yet many are able to derive an in-game advantage by being able to edit add-on code, write their own weak auras, etc.
Having knowledge in coding and/or programming shouldn’t be a prerequisite for obtaining an advantage in the game, and I imagine most don’t like this about the game.
Having said that, yes, it takes minimal effort to just download the add-ons and utilize them with their default settings, and quite honestly, this is more than enough for most to play the game and enjoy doing so, however, it would be disingenuous to deny that players with the above knowledge have an advantage over those that do not.
As another example which is hopefully relatable, weak auras can often be incredibly complex, and as a non-techie boomer, if I can’t find the weak aura(s) I need via a google or two, then I just won’t have access to them, which is at least a minor disadvantage in terms of how effectively I can process information compared to the next guy [e.g., Bob has icons flashing around and a voice of choice going off every moment that someone presses a cooldown, whereas Fred has to hope that he noticed relevant abilities being used by squinting at various icons amongst a sea of other icons].
At the end of the day, I don’t think being able to properly configure and/or optimize the various [essentially required] add-ons, including weak auras is going to matter below rank 1 range, but an advantage is an advantage, so I can understand why many (if not most) players would prefer for there to be a simple uniform interface and/or means for which relevant information is conveyed to each player.