You can certainly do that, but where should that blue line be drawn? What doe the coordinates mean? You need to know what the current transformation matrix is to make sense out of the coordinates. With a few lines of code in an Acrobat plug-in, you could add functionality that takes these PDF content stream operations and adds them to the content stream, but depending on where you add them, you will get different results.
It's much easier to let the user pick two points via the user interface (click on the start point and drag the mouse cursor to the end point of the line) to specify where exactly the line should be.
If you really want to inject code into a content stream, you can do that today. I use the Enfocus Browser for that.