If I receive a fairly heavy volume of notifications, and switch between spaces frequently, I would prefer to see them all remain onscreen regardless of what space I am currently in. In the Exposé & Spaces preference pane you can choose which space a particular application appears in by default, and one of the options is "Every Space." That is the behavior I'd like to see Growl exhibit. But because Growl runs in the background and isn't recognized as an application, I can't make that assignment here.
It's not a super-important thing, but it would be a nice touch for a Leopard-optimized version of Growl in the future.