What does setting LSUIPresentationMode to 1 or 2 do? While reading "Is it possible to hide the dock on a per-application basis?", I noted that according to the documentationLSUIPresentationMode, setting LSUIPresentationMode to value 1 or 2 (in an application's Info.plist) should mean that "system UI elements in the content area of the screen are hidden", and (it is implied) that this includes the Dock. 

In practice setting this key to values 1 or 2 appears to do nothing (neither the dock nor anything else disappears), while values 3 and 4 behave as documented.

Do values 1 and 2 ever have a useful effect?  If so, what is it?

(I have tested this setting with a variety of different apps, including Aperture, Chess, Stickies and Emacs, all under OS X 10.6.8.  The behaviour was identical in each case: settings 3 and 4 behaved as described, while settings 1 and 2 appeared to have no effect whatsoever.)
I believe those keys are intended for cases where you're writing an app that will use its own full screen mode (such as a video player or game). To see any effect from changing them the app likely needs to be coded appropriately, not just have the key set in its Info.plist.