Comment 8 for bug 1084885

I concur that it is likely a rarely used feature (tho "single digit" seems low). However I think breaking compatibility with a long history of Xdm is an unwise move. To get the best of both worlds (compatibility & feature set, and performance), I would suggest a config option for lightdm. The main app can push that config option into a env var before it calls /usr/sbin/lightdm-session. Then /usr/sbin/lightdm-session can test it. Something like this would work for me:

C code (pseudo code):
   if (config.xrdb_cmd) {
     setenv("LIGHTDM_XRDB_CMD", config.xrdb_cmd, 1);
   }
   ...
   exec("/usr/sbin/lightdm-session" ...);

Shell code (/usr/sbin/lightdm-session):
    xresourcefile="$HOME/.Xresources"
    if [ -f "$xresourcefile" ]; then
        echo "Loading resource: $xresourcefile"
        ${LIGHTDM_XRDB_CMD:-xrdb -nocpp -merge} "$xresourcefile"
    fi

The result is that for all the "simple" users, it's as speedy as ever. For advanced users, its a one time config change to lightdm, and the run-time cost of cpp. Using grep is ugly. Using "test" would be worse (assuming you are proposing sucking the resources contents into a shell var in order to look for something in it).