I'd like to suggest hiding the tabs above the drawing window in the case where only a single drawing is open. The tabs take up a fair amount of vertical space.
Personally I prefer having the tabs there for reference and I find it helps my work flow.
But you could add your idea as a feature request here;
(you will need to register to use it)
Suggestions for more vertical space
1. If you dont need the rulers, you could hide them - Edit > Application preferences > Graphics view then check/un-check rulers. (you will need to re-start QCAD for this to become effective!).
2.You could un-dock the command line and re-dock on the right-hand side.
Hope this helps.