Version: (using Devel) Installed from: Compiled sources add login and cancel/clear buttons Login -> because this way it is more explicit, more like KDE dialog and more comfortable for users to see the "confirm" action button Clear (or cancel) -> if you entered the login/password values but for some reason you would like not to login right now (for example your boss is calling you) -- it is faster not to login, instead of logging in and locking the session and it is faster to click on "clear" button then manually erasing those two fields
i suppose you are referring to the themed greeter. providing a login button is an option, but it is ultimately up to the theme to do it. the clear button was removed from the standard greeter years ago because it adds pointless clutter - escape works just as good. this is a view suggested by usability experts, iirc.
Escape key -- it works, but the problem is that there is no other place where escape clears the edit box. It could not be "clear" button, the black #> widget used in editboxes in KDE could be used. Anyway both functions (commit, clear) should not be hidden.
PS. Usability -- from my experience, I always hear from users questions "and now what? press enter?". They pressed it million times, but they are still unsure what to do if they don't have clear button for it. Escape key here is beyond their capabilities of memorizing all mechanisms under-the-hood of KDE.
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/