I'm not too horribly appalled when I see a dog riding in the bed of a truck, only because I literally see it everyday. I live in a town where most everyone believes its fine. All of my uncles let their dogs ride in the bed.
So really, I'm pretty much against it and in a way I'm not, but I don't make a big deal about it. I've grown up around it. I have never seen it done in a big city, and thats probably why I'm not completely against it. The town I live in is extremely small, so the speed limits don't get very high. I know, I know, accidents still happen, but you take a risk of dying or getting injured just by driving yourself.
I'm also not worried about these dogs jumping out. If they ride in the bed of a truck, it normally means that they have done it from a young age, so they are basically trained to stay. Especially my uncle's dogs. They will not budge from the bed of the truck unless you give them a specific command to do so.
Personally, I wouldn't let my dogs ride in the bed of a truck, atleast Skylar and Ethan. They both freak out if they're in a moving vehicle without a human right beside them. Frisk, on the otherhand, has ridden in the back of a truck with a few people (only on a gravel road, though). He knows not to jump out untill told that he can come. He won't even raise up on the side of the bed. I probably wouldn't let him ride in the bed alone, only because he is small enough to fit on someone's lap if needed.
EDIT// I also wanted to state that most dogs I see riding in the bed, are only riding there because the cab is completely full. Face it, most older trucks (you don't see anything newer around here) aren't extended cab or anything, so it only holds 3 people.