Why would author list off a bunch of important things the thief class does and say it's useless? It's always like this.
Fantasy manga where a particular character class or profession is called useless are a dime a dozen, that's for sure. Most often it's not explained in any way. My favourite is healers being useless. Japan itself practically worships doctors, physicians, who are at the top of the list of desirable men in shoujo/josei manga. Yet big brain light novel authors can decide that healers are useless. Because everybody absolutely wants to die or remain disabled for life after getting wounded or sick.
At least this series makes a point of the MC presenting himself poorly. For all we know, his class might not be shunned at all, he personally just is due to his own behavior and lack of taking care of himself. He would then blame his class for it. But then again, maybe his class really is disregarded and that eventually led to Harris giving up on himself. If that's the case, he should move to a difference city or even country. Things ought to be better elsewhere.