Modern kernels have the ability to schedule groups of processes instead of just individual ones, meaning that you can get a more fair distribution between e.g. users. It needs help creating these groups though so we should consider creating a new one for each session we start.
Some interesting info:
Might not be needed anymore. systemd automatically does this via pam_systemd.
Here is an example of when pam_systemd isn't working well enough with ThinLinc.
See bug 44 and bug 7304 which has variations on this theme.