I think for cards like Xiara, it makes sense for certain cards to not be prelevant in the tournament scene, I have one dumb Corruption/Order deck which can generate Militia. I don't mind there being cards that are just a lot weaker than others, since Xiara at least provides a cool/interesting ability.
On the other hand, it makes sense for a Legendary card to be strong; but I don't think it's absolutely necessary if they bring in a unique ability, or something on a huge scale. I believe that some cards can be unplayable in tournaments, because there might be someone who wants to use it in some dumb just-for-fun casual deck. Since there are more than just ladder grinders on this game, it makes sense to keep weird/bad cards around, so those people can just mess around.
I'd actually like to see weird game modes appear eventually, although obviously there may be other priorities. MTG has pauper, and it'd be cool to see that in this game when there are more commons.