Comment 5 for bug 774644

This bug is a little worse that it may seem. If you use slim to launch unity then it subtly breaks compiz so that opengl doesn't work properly and all operations and slow - further you will only notice this once you reboot your machine after setting up slim because plymouth has already been killed.

I'm not sure how common the use case of wanting to replace lightdm / gdm but keep unity on a ubuntu box is, but I'd imagine it might common enough to merit this bug being pushed.