It appears that kdeplasma-addons requires Akonadi as it currently stands. This seems to imply that USE="semantic-desktop" and related packages are required. Reproducible: Always
Created attachment 269029 [details] build.log
Mathew, unfortunately this is a known issue. Upstream keeps breaking the build without akonadi or, if you want, forcing semantic-desktop. *** This bug has been marked as a duplicate of bug 353730 ***