When keyboard layout module was redesigned for KDE 4 it felt like indicators of sorts were to be done in widgets/applets instead of tray icons, so keyboard layout indicator applet was created. It does allow greater flexibility that systray icon on where to put it and how to size it. I learned though that the keyboard daemon can't control that applet much (e.g. hide/show automatically) so when multiple keyboard layouts are configured there was no way that indicator applet can automatically appear (like system tray icon does).
So the system tray indicator stayed in with the hope that the user could choose whether to use tray icon or keyboard layout applet (or both).
It seems though that on some systems both keyboard layout indicator applet and system tray icon appear together which confuses users: https://bugs.kde.org/show_bug.cgi?id=264886
Also the user can place the applet on the desktop and when that applet is clicked to switch keyboard layout the focus goes to desktop window which we need to ignore in "per application" and "per window" mode which leads to defects like this https://bugs.kde.org/show_bug.cgi?id=269661
One of the solutions is to remove keyboard layout indicator applet and leave only system tray icon. The opposite solution: only leave applet and remove systray indicator does not seem right as it'll force the user manually add keyboard indicator applet after configuring layouts which most probably will confuse a lot of users especially novice.
There's of course always third option - leave it as it is right now and if the user added both indicators let them remove one. :)
I learned over the years that removing features no matter how redundant they may seem does upset users a lot so I created little poll (at the top of this blog page) to check which indicator people use to at least have some idea about potential impact on users.