It's a good question. Let's say you're like me, as many are. A regular to avid cyclist, who has a kid or two to haul sometimes, some groceries too, and a limited budget. You've already got your performance bike(s) and a bike you can nominate the errand/daddy bike. Should you get a cargo bike from $1K to $3K? They do look sweet.
Or should you spend less than $500 to get a kid trailer? Not so flash. Frankly, I think they are overbuilt and unstreamlined all of them, and none come as a more efficient single-wheel trailer. Yet, when you don't need it, it's not there. Did I mention the fraction of the price? Or that there are often second-hand sales? The kid-trailer also acts as a great decoy to fuckwit drivers - even full only of cargo they may try not to hit a child more than an adult.