When users have many tabs opened, the more comfortable way to display them is vertical list with horizontal text labels. But at now QT have no way to do this.
This is popular way to display tabs in other apps:
Firefox: http://www.thetips.org/wp-content/plugins/pe/pics/086fd_vertical-tabs.png
Google Chrome: http://www.3dnews.ru/_imgdata/img/2010/07/23/595620/chrome-vertical-tabs.gif
Safari: http://limi.net/media/tree-style-tabs.png
KDEPIM mobile: http://dot.kde.org/sites/dot.kde.org/files/kmail_mobile_maemo1.png

At now in QT there are no way to create tabs like this, so developers use other widgets for this. See in Kate: http://wstaw.org/m/2011/11/02/plasma-desktoprD9066.jpg
KDEPIM: http://wstaw.org/m/2011/11/02/plasma-desktopKB9066.jpg

So will be much better if QT release the internal vertical tabs style in QTabWidget.