Comment 19 for bug 879793

You will find gnome-shell uses many of the same processes as Unity, including nautilus, so will share some of the bugs too.

Why Gnome/GTK developers can't look at .xsession-errors and eliminate all the bugs/warnings in their code before release I don't know... But that's the way it's always been in my many years of using Gnome desktops. The log has always shown some bugs and warnings are still present.

I agree that bugs like this significantly tarnish Ubuntu's image. However according to the guidelines, importance of Low seems to be about right:
https://wiki.ubuntu.com/Bugs/Importance