The Scriptable Finder should be looked upon as a first release of product whose capabilities will increase and improve.
Control Panels are a special class of Finder objects, not applications. Currently, the General Control Panel has no scripting capability. Applications such as Desktop Patterns can be implemented with their own scripting capability. A few Control Panel such as the Views control panel have some scriptable features, and to a limited extent, so do Sharing Setup and Users & Groups. Monitors settings have been addressed through Scripting Additions.
Since Control Panels are Finder entities, their scripting definition have to be embedded into the Finder's Dictionary. The current presence of scriptability of a Control Panel function is certainly related to the probable frequency of use. For example, how often does one change the Date & Time or set the names or colors on Labels as opposed to changing a folder view or starting file sharing? Likewise, it probably would not offer a lot of additional functionality to be able to script the MacTCP Control Panel.
With respect to toggling "Show Desktop in Background", AppleScript does not provide that option. Like your Mouse Speed, this╩function falls in the category of a user setting it once and leaving it that way.
The many setting within Control Panels is where other OSA compliant extensions, like QuicKeys or Frontier, can step in and assist. QuicKeys and Frontier use a different model of scripting and each has weaknesses and strengths compared to AppleScript. A strength of QuicKeys is that it can script almost any application, desk accessory, or Control Panel, whether they are AppleScript-aware or not.