One of the reasons to use :extend t faces, IMO, is to make it easier for
the eye to delineate horizontal chunks (e.g. "hunk headers", "added
lines", "removed lines" in diff-mode).  The unfontified prefix inserted
by adaptive-wrap makes the overall result visually confusing to me.

In these situations I'd like the wrap prefix to have the same background
color as the extended background: see third screenshot.