"But," you might say, "what percentage of the population are actually experts? The odds are probably better that he's both worthless and a pedophile!" And if you're saying that, congratulations, because you share the same worldview as the producers. So let's say the hitchhiker isn't an expert at anything, but did work at a Walgreens when he was in college years ago, and thus knows what antibiotics to steal and what dosages to use them in. Or maybe he was nothing but an unemployed hipster, but he was an Eagle Scout when he was a kid and thus knows how to tie every kind of knot, how to identify every kind of edible mushroom, and how to skin every kind of game. Maybe he's simply a really good cook. How many people do you know who are truly worthless? You can learn a bunch of useful shit merely working at a convenience store for a month.
Hell, maybe he's just really funny, and thus good for morale. You don't think that would be helpful in a world in which 90 percent of the conversations are about how they don't know if they can continue under the weight of their crushing despair? If so, it doesn't occur to anyone in the world of The Walking Dead. It doesn't occur to them that any fellow human might have something useful to offer. The central theme has always been that in a world where society has broken down, only the most cold-hearted, murderous badasses survive. The rugged individualists who don't need anybody.