It could be interesting for those who have multiple Address Books and need a way to distinguish them at a glance, or those who want to organize their books, or even those who simply want to customize and want different esthetics to their Address Books!
I would only suggest some small things, nothing too fancy...
Perhaps the possibility of having different preset icons, related for example to work (a suitcase?) or family/friends (a group of persons, maybe?) ... Those icons would be in the same style as those used throughout Thunderbird.
Maybe also a possibility to color the name of an Address Book, like how tags color the name and variables of an email in list view...
I don't expect this to be given much priority: I am very aware that there's still a lot of work going on Sync and other features, I can see the size of the release notes, I fully realize there's truly other things way more important than what I propose and I know that time and resources are at somewhat of a premium.
Maybe someday, who knows...