I suppose that some may think of it like that, but luck is a powerful skill. There is chance involved minutely in many things and luck can change entire situations with ease.

[She thinks back to some of the things shes seen Vriska do with her abilities]

Besides, what may be considered worthless in one society may be considered useful in another. Although human society is much more lenient with the definition of and consequences for being worthless, I think that you probably would have been found useful for your Luck by troll society.

[Kanaya furrows her brow at that train of thought. She thinks the Condesce would have either tried to use his luck as a weapon or cull him if proving troublesome.]

Probably not for the right reasons, but reasons nonetheless.