I often hear people bash the standard UI, which I admit, is not the greatest, but its still usable.
During BC while being in a top guild on my server, I used the standard UI but added Clique and Grid to have my groups smaller on the screen, but aside from that, I used minimal key bindings, and very few add ons to help, and I was never low on meters for healing. This is also regardless of which class was the OP healer at the time. We never wiped due to my lack of a custom UI on my part, or could not down a boss because my output wasn't high enough, in fact, there were people that used a custom UI that struggled in comparison on meters.
This also led into my raiding during Wrath.
I never understood the amount of people that act like the game is unplayable without swapping your entire screen set up.