This is good feedback, xyz. Signatures are meant to be used as something of a self-promotional tool, and while most people use it to link their cube lists, that certainly isn't a requirement. Because we don't really want sigs that link to random blogs and such eating up a lot of real estate in a forum discussion, we've limited them to show up only on a user's first post per thread page.
In the upcoming release of Xenforo, they'll be adding some more information that can be displayed underneath the user's avatar, that's formatted kind of like a 'badge'. I think that would be a better place for people to link their cube lists. This was something that James Stevenson suggested in another post, and it should better address the concern of "what does this guy's cube look like". So in your user profile, there'd be a Cube List field, and if you filled it out it'd show up in your badge.