I think white is a very important 'color' in the garden
Yes, I agree. Whites make the other colors more brilliant.
I also (sadly) agree with you about some white flowers looking bad as they age. I have some white Pelargoniums which I have to constantly deadhead, as the flowers turn brown. Similarly, with a white Drakensberg daisy, the white flowers get brown streaks.
The white irises, thankfully, don't have that problem.
When I think of it, during iris season the irises I most look forward to are the (Moon Garden) whites, a seedling iris, 'Navajo Jewel' (not turquoise but very beautiful imo), and the two dark purples. I often wonder what a bed of just white and dark purple irises would look like, though I suspect a third color, a pale yellow or something pinkish, would be needed to make the whole thing really shine.