I don't know why vitamins in water wouldn't be good - maybe you/they are thinking about things like potential for bacteria growth, etc., but that would be preventable.
I do prefer, however, not to use vitamins in the water, but it is a personal preference - just reflecting the way I keep my birds. I like to keep larger water dishes available for my birds so they can bathe whenever they feel like it. (That also means changing the water often!)
Because I would need to use more vitamins to make a correct dose in a larger water dish, it is a lot more expensive delivering vitamins that way.
While it's probably fine, I don't know about the wisdom of birds covered with vitamin water.
Some vitamins can stain the cage and surrounding areas. When I first started with birds, I was using Nekton vitamins in their water. Wherever that Nekton water dripped or was splashed, seemed to develop a yellowish stain that was difficult to remove - especially plastic and wire that wasn't powder coated. Still use Nekton - LOVE it - but now sprinkle on food.