I'm not sure why you guys are wanting backstories for the characters, the entire point of the Minifigures line is to provide basic characters that can fit into any variation of their basic theme - even when their online bios give them some sort of backstory, it's very light and flexible. For example, the Evil Knight has worked for Basil the Bat Lord and Cedric the Bull. Seeing as these are the characters you'll be playing as, and the worlds are based on actual themes like Pirates, I'm not sure if the mission giver NPCs will even be from the Minifigures theme at all, they could be from the more standard themes for all we know. Even if they aren't, it doesn't mean Funcom won't be coming up with their own story to link everything together.
With MMO's I kinda want a reason to 'exist'. Being thrown into a MMO without knowing why you are what you are or if you have a place in the world... is not very... captivating. It's just like: "I'm a minifig, I wear armor and kill stuff.... why am I playing this again?... Oh! Everyone! check out my hat collection!".
I guess kids don't really care about this though... and that is really who their target audience is.