This is a code limitation - handling just modifier keys is different to handling a key combination including modifiers, and I didn't think that anyone would want to use just Super as the summon key.
You also end up with odd problems like Unity (and GNOME Shell) is having, where only Unity gets keypresses which contain Super.
It'd be possible to implement modifier-only keybindings, but I'm not sure how much effort it would be. I'd accept a clean merge request or patch implementing this, but I'm not planning on implementing this myself in the near future.
https://bugs.launchpad.net/do/+bug/1093004