This is true, but i find when opacity is used (especial on dark backgrounds) colours can become washed out due to the transparency (I tested this in photoshop) and assumed the same would have the same results using the opacity property. Your not going to get the true colour using opacity where you would if you lightened the image (without opacity applied) which will retain it's true colour.
I did initially create a forum-new icon, (referring to the node icons i submitted here) and then duplicated the group (icon) and lowered the opacity and as predicted the colours wern't true, nor do you get a true effect on the layers (like the shine, border etc) so I went ahead and just lightened the colours. Sure, what your proposing wouldn't be bad on light backgrounds, on the otherhand, on darked backgrounds using lowered opacity the colours and other effects don't hold true due to the bg colour showing through.
I guess this might be one of the reasons why opacity wasn't applied, just a guess.
Edit: Then again, they could have already lowered the opacity in the graphics editor they used. There's not way to tell whether they lowered the opacity or retained full 100% and simply lightened the colours.
