It just has to be passionate. If you can also make it useful then even better.
The reason why you think it should be useless is because otaku in order to do what they are passionate for, they tend to a little, eccentric, weird, strange and to neglect things that "normal" people do. e.x. not study in order to play mmorpg all night. Of course that doesn't apply to all otaku but it's a fact. Otaku also tend to know so many things about what they like and loose the knowledge of other useful things.