Comment 5 for bug 172390

> Why ban it completely?

Because KDE works at the user level, and not at the user+session level.
It supports multiple desktops, but a single session per user.

At login time, KDE sets up specific resources in $KDEHOME to ensure proper behaviour throughout a session (this includes reading/creating files, proper logging such as kdebugdialog, and cleanup). This setup is not supposed to be run several times asynchronously.

In addition, some resources and programs are single-instance resources (e.g kontact), and have a locking mechanism enforced by files in $KDEHOME.

Here is what will happen with single-instance resources:

my_user logs in session 1, starts kontact
my_user logs in session 2 -> cannot start kontact in this session

In other words, parallel sessions of the same user can interfere pretty badly, including denial of access from one session to the other, messing up files in $KDEHOME, and bogus logging.

Cheers,

Philippe