From the website:
> Since extensions are created outside of the normal GNOME design and development process, they are supported by their authors, rather than by the GNOME community. Some features first implemented as extensions might find their way into future versions of GNOME.
For another system monitor (this one attaches to the panel, so it's always visible, and is plenty configurable), check https://extensions.gnome.org/extension/120/system-monitor/
EDIT: GNOME's stance on extensions might be changing, see https://bugzilla.gnome.org/show_bug.cgi?id=685744
I am one of those (apparently few) people who actually agree with the Gnome approach of less configurability and options. (Right up until one I use is removed of course!) As standard I do not need a large choice of system monitors, workspace managers, window switchers, volume menus etc. I need one good official one, that works out of the box and is still present as Gnome does its biannual update.