Why isn't SnoopDockieDock a MenuIcon just like the battery and clock icons in OS X 10.1?

Two reasons: first, I haven't read any documentation on how to make such a thing so I wouldn't know how to write one. But most important: I don't think it is very handy. I usually try to find something in the Dock only to find that the item I'm looking for isn't in the current Dock but in another one. Then all I have to do is (since my mousepointer is already at the Dock) use SnoopDockieDock to select the item from another Dock. If SDD were in the menubar, I would have to leave the Dock first and move the mouse pointer all the way to the menu bar to switch the Dock.

Why do you use both a 'launch from' and a 'switch to' part in the Dock instead of combining the two?

The reason is that a lot of people found it cumbersome to have to open the submenu of a particular dock first and then select the switch option. Since it isn't possible to associate an action to a menuitem which has a submenu attached to it, I couldn't use the DockName as a switch button. So I chose to make two seperate parts.

Why don't you use tickmarks to show the active Dock?

This is because Apple's APIs do not yet support tickmarks in a Dock menu. Also, it doesn't support (yet) pictures in menu items. You probably know that when you put a folder in the Dock and right-click on it, that you see the contents of the folder with names AND small icons. Unfortunately, Apple still hasn't provided this capability to programmers but they say they will fix it in the future. As soon as this is the case, the SnoopDockieDock menus will not only show the names but also the icons of each item in a Dock.