I never noticed this before, but it was brought to my attention in the PitBull thread that widgets for disabled options don't display their descriptions in a tooltip. Is there a limitation or design consideration that resulted in this behavior?
(I had made the suggestion that the author append the reason an option is disabled to its description, so that users would see it in the tooltip.)
I think it's more of a side effect of no mouse interaction than a choice to not display tooltips if the control is disabled. I prefer the no mouse interaction myself, and I agree with being as consistent as possible with the default UI.
Maybe you should make an addon that overrides the behavior of all the checkboxes?
With my degree of Lua proficiency, I'm afraid I'd be likely to introduce far worse side effects than I'd "fix" in the process. :)
Edit: My Lua proficiency isn't really that bad. It's a straightforward and easy to learn scripting language. And there are plenty of great resources for me to go to for Lua help. It's just understanding exactly how (and why) WoW does things that gets me lost at times.
For example, I realize that enabling the mouse on control widget frames would cause textures and fontstrings in their "HIGHLIGHT" layer to be shown on mouseover. I'd wager that these regions don't get changed when the control is set to disabled, since disabling the mouse is a shortcut to ensure they don't get shown anyway. Simply enabling the mouse could cause the unwanted illusion that the control is interactible while it's disabled.
I'd guess that click events might go through, too, and the handlers for those might not check if the control was disabled before changing some state. (After all, why should the handler bother checking, since the control is assumed to be unclickable when disabled.)
I can imagine the efficiency gained by disabling the mouse on disabled controls. The complexity and potential for serious error in trying to make these things work according to my personal design whims is terrifying to me. :P