The new version of sddm-0.13.0-r2 has the use flag of systemd backwards: installs sddm service if not using systemd , and doesnt install if using systemd.
the code in the ebuild in question:
It's fixed in -r3.
*** This bug has been marked as a duplicate of bug 576320 ***
Sorry, didnt' found the duplicate before submitting.
I did a quick search for sddm and it didnt gave me any bug report mentioning this problem.
Thanks for fixing it :)