MATE can work with or without systemd, but it doesn't detect it properly. It basically checks for logind and assumes systemd is running.
So when you switch to full systemd, it works in systemd mode.
And if you were to remove libpam-systemd and systemd, it would work fine in legacy mode.
The problem in LMDE is that we have logind but we don't use systemd itself...
So anyway, the easiest solution was to force MATE not to use systemd. That means people don't need to do anything but to accept the update to mate-power-manager 1.6.2-2+lmde and logout/login again.
Now.. for those who want to run systemd instead of init, please stick to mate-power-manager 1.6.2-1+lmde and hold onto it.
I've kept both versions in the repositories (1.6.2-1 in import, 1.6.2-2 in upstream) so everybody's happy