This is my thoughts as well. I don't see a need to jump through extra hoops to default the signature to an off state for users that want to use signatures. The reason why its my signature is I want to set it and forget it, not need to remember to check the box every time I post it.
As mentioned, you can disable the view of signatures by changing the setting in your user control panel > Edit Options section.
I do like the idea of perhaps having it unchecked on each subsequent post. That would require a code modification for though, so I doubt we'll see that happening.
header('HTTP/1.1 420 Enhance Your Calm');