Usability of transparency in navigation menu

I have seen a couple of websites using some css properties to make the navigation menu semi-transparent. I assume this has to do with giving the user a more continuous feel to the page scroll, but it seems to me that it is more distracting then creating a better user experience. I believe there are more downsides to positives, but are there any real usability design principles behind this?

Menu appearance when focus is on page Menu appearance when focus is on menu