Red Bull,
The answer, in a word, is simplicity. When lighting and target conditions allow a clear view of the outline of the sights, anything else becomes a distraction. Plain, black sights provide the clearest sight picture under ideal conditions.
But as you've noticed, black sights on a dark target don't give much contrast, especially in dim light. That's why my personal preference is for some sort of dots or other marks to align when the outline of the sights isn't visible. Having the extra "doo-dads" does slow me down some in good conditions, but at least I still have something to align in situations where all-black sights would be useless.