Bescause excessive transparency impedes legibility, and the primary goal of a menu is to expose text-labeled commands to the user, so anything to improve legibility is good? I have the same criticism of Vista's horrendous title bars and translucent windows (the gadget pane is the worst offender).
People said the same thing of the translucent menu bar and Apple gave into them. They now have the option to turn the translucent menu bar off. But they made the menus themselves much more opaque without giving those of us who liked it the option of keeping them as translucent as before.