There doesn't seem to be any visible loss in quality when keeping the quality at 100%. How does Photoshop treat save for web differently from regular save when it comes to JPEGs?
Photoshop uses a different compression algorithm when saving images for the web. Quality is lost when doing it this way rather than saving the image as a JPEG normally (as in going through Save As etc). When a website is loading an image, it's best for it to be smaller in file size so it can be delivered to the user's computer quicker. You must remember as well that photographers use Photoshop as well, and I feel Photoshop is geared more towards the photographer or people who work in print than people who work on websites. So in the photographers best interest, it's better for their JPEG images to be bigger in filesize and therefore better quality.
I would suggest however that you use Fireworks when saving images for use on a website. Fireworks' compression is far better than Photoshop's compression with no comparable loss of quality between the two.
A simple comparison can be seen here http://blogs.popart.com/2009/02/why-aren-t-you-using-fireworks-to-compress-images/