there are several reasons for the change:
* we want to have only one DM for all flavours because of the needed manpower
* kdm is a pita to customize
* kdm is dead - see
http://aseigo.blogspot.de/2013/03/logging-into-plasma-workspaces-2.html* lightdm works good with systemd 204-x
* i don't like kdm - see reasons above
To be clear: lightdm is a compromize - it's mature, stable, scriptable, looks good - bad thing is: Canonical is behind it, so we should have alternatives for the future. This future can be eventually sddm - but it is in a very early state.
On ne saurait faire une omelette sans casser des oeufs.
By the way - Three hours before the release of the rc1 we decide, that gnome should come with lightdm too because of major faults in the combination gdm3/systemd.
- So we think, that lightdm is the best solution for now. If anyone don't like lightdm, feel free to purge it and go with siduction-settings-gdm3-december, siduction-settings-kdm-december or siduction-settings-slim-december, but keep in mind that we are not able nor willing to support or develop these DMs with full power. If there are bugs - we would be glad to accept and apply patches.