Whenever I use GNOME terminal to dump program output to stdout (e.g. for debugging purposes), I encounter the problem of post-processing the dumped lines. 
Of course one could redirect the output right from the beginning, but I frequently tend to forget it and restart things, which is cumbersome.
Selecting everything inside the terminal manually (using the mouse) is not a real option for large text dumps (selecting and scrolling seems to be quite slow in GNOME terminal). 
I assume that a menu entry "Select all" below "Edit" would be easy to implement, helpful to many, and fit well with the other items in that menu bar.