One of the reactions to my note on how the window manager uses the double-click time as a way to tell how good your reflexes are is bemusement, well okay, not so much bemusement as outrage, over why we need a menu show delay in the first place.
Imagine, you're navigating a deeply-nested menu hierarchy and then you want to move to one of the items in the most recent fly-out, but instead of moving the mouse directly to the right then down, you move it ever so slightly diagonally down. Boom, the entire menu collapses and you have to start over. There's a place for maddeningly fiendish mouse dexterity games, but trying to create a pivot table is not it.
I run into this problem all the time on the Web. Web site designers forget to incorporate a menu show delay, resulting in frustration when trying to navigate around them. For example, let's look at the navigation bar on the home page of The Discovery Channel. Hover over TV Shows, and the menu appears. Suppose you want to go to Koppel on Discovery, but instead of moving the mouse straight downward, the way you hold your arm on the desk moves the mouse in an arc that happens to swing to the right before it arcs downward. You touch TV Schedules and your navigation is screwed up. You have to start over and make sure to move the mouse exactly straight down.
This phenomenon is even worse for sites that position their submenus as a horizontal bar below the main navigation bar, such as EVA Air or NBC. On the NBC site, for example, hover over Schedule and a band appears below the navigation bar with more options. But you can't move the mouse diagonally to, say, Pacific; if you do, you'll accidentally touch News & Sports and the Schedule submenu will be disappear. If you just move and click in one motion, then boom, congratulations, you just clicked on Access Hollywood. You have to carefully move your mouse straight down (being careful not to touch anything else that might open a different menu or cause the existing menu to auto-dismiss), and then straight sideways (being careful not to accidentally move it upward three pixels, causing the secondary navigation bar to be replaced with a different submenu). It's like its own miniature mouse dexterity game. But one you didn't elect to play.
The folks over at TechNet Magazine did take the menu show delay into account, though it's still not as long as I'd like. Hover over the word TechCenters in the navigation bar, and a submenu appears. If you move toward Script Center and happen to touch TechNet Magazine ever so briefly (like one Planck time), the TechCenters menu stays up. On the other hand, the menu animation is abysmally slow if you use Firefox, so one step forward, one step back.