I guess it greatly varies how well one would perform with or without addons, based on the size of your monitor, UI scale, and simply what you are used to.
I assume you are using the in-game raid frames to heal? The only difference between Grid and the standard raid frames is that Blizzard didn't make grid. Using something Blizzard didn't make doesn't make it lazy, or worse. I play on a 1900x1200 screen with my UI scale set to 50% of normal. I cannot use the standard interface Blizzard designed, cause the gaps and distances between sources of information are too great.
However, I agree to a great extent that addons are useless and vain. Yes, they do make you lazier. I could probably do quite well without almost all of my addons. I have said this before, the only
addon I could not live without, is grid.
All this I can see with my eyes on the standard UI, so I fail to see what it is I lack in the form of add-ons to perform better?
Well, for instance, I don't want my raid frames taking up the majority of my screen. With the standard frames set to the tiniest of settings, it's still three times as big as I have Grid set to. Secondly, I want to be able to show exactly what buffs I want to see. I don't want to know whether I have Grace, Inspiration or Echoes of Light up. As far as I know, the standard UI doesn't let me select what I want to see. It just shows me everything.