In my opinion, when middle clicking on a single task in the tasklist it should close. Just like middle clicking a firefox tab will close it. It is extremely helpful.

Other information:
This patch for libwnck in tasklist.c keeps the current click on a group of tasks goto next task but when clicking on a single task it closes it.
:
3694,3699c3694
<         }
<       else if (event->button == 2)
<         {
<       	wnck_window_close (task->window, event->time);
<       	return TRUE;
<    	} 
---
>         } 
Thanks for your time.