Looking now at the Typekit site (where the font comes from) I see that Adelle, indeed, is not recommended for body text by Typekit, only for headings.

The Typekit site offers some short explanations about what these recommendations mean:

Fonts recommended for paragraph text: “Recommended for continuous text at small sizes, these fonts possess a generous x-height and are manually hinted to perform well across operating systems.”

Fonts recommended for headings: “Recommended for short text at medium to large sizes, these fonts are generally more decorative and often served with PostScript outlines for improved rendering.”

A good place to start reading if you are interested in this is a post at the Typekit site again, part of a series on web fonts and font rendering:

http://blog.typekit.com/2010/12/17/type-rendering-review-and...

It includes this:

“All of the fonts tagged Paragraph have been manually hinted to look great on screen at text sizes in every major browser/OS mix. Use these recommendations to make informed decisions about how well a font will render, knowing that they are backed up by Typekit’s font diagnostic tools.”

More generally:

I have found that often—I’m tempted to say more often than not, but I have not the data to back this up—designs using web fonts are worse than they would be without web fonts. When you consider how complex the issue of good rendering across devices and operating system is, that’s not suprising. It is interesting though that even professionals—even people who teach web design and who write books on web design—make such mistakes.