Currently the menu panel applets have an inactive region between the clickable region (that opens the menu) and the edge of the screen, this makes a fairly small target to aim the mouse at.  Currently the show desktop and trash applet do not have such a border, so are easier to use because when placed in the corner they are effectively "a mile high and a mile wide" (i.e. the target you are aiming for effectively continues off the screen forever.).  
I suggest that the menu applets have similar functionality so that they are in effect a mile high/wide.  The very corners of the screen are the easiest places to locate with the mouse.  This improvement would make improve the usability of the menus especially for those who have have problems controlling a mouse so accessibility would be improved.

Other information:
For a discussion of this see http://www.joelonsoftware.com/uibook/chapters/fog0000000063.html which deals with  menu panels at the edges of the screen and the placement of the Windows start menu from a usability perspective.