from the login screen settings> users tab, the show list box under users is checked, but when at the actual login screen, a user list never shows up, and you have to type in the user. This is with the default kubuntu Ariya theme on kde 4.8.3
*** This bug has been marked as a duplicate of bug 223577 ***
This bug isn't about whether or not kde should have a user list, it is about the option for displaying a user list that already exists in the settings not working
I can't reproduce it using the Oxygen kdm theme. It might be a problem of that "Ariya" theme from kubuntu.
kdm reached it's end of life with Plasma 4, whose last bugfix release was last August 2015, we are sorry we did not get to fix this bug. We encourage you to try Plasma 5 and its recommended display manager SDDM and it's control module kcm_ssdm. And please do not hesitate to file bugs against it if you find any. Sorry again we could not fix your bug.