As long as it is not a flat bed & the dog is trained well I don't really see a problem with it. Using a restraining device is even better. Sure riding INSIDE the vehicle is probably safer for them. I just feel that if the dog likes it and is trained not to stand on the edge or anything like that, that it is probably not too much worse than riding unrestrained inside a vehicle.

Besides the dog loves it. I don't have a pick up but I let my dogs have their heads out the windows in my car & van.

It might not be the safest but you run a risk no matter what way you deicde to travel with your dog.
If you dog is crated and your vehicle is crushed badly that may cause injury & or cause your dog to become trapped.
If you have a divider in the back of your station wagon or suv and you are rear ended your dog may get more injured than if he was in the back seat, or he may even get trapped. Without a crate or divider the dog at least has the option to jump into another seat. And even with the divider there is still a good chance your dog can go through a window if you get into an accident.

I personally believe that as long as the dog loves it and is trained that it is fine. Heck I drink a beer here & there and the like. Some people climb mountains & the like. Why can't dogs do something a little "bad" or "risky" that they enjoy?
I know I'd rather die happy with more experience than sad & sheltered and I honestly feel that my dogs feel the same way.

All in all there is a risk with EVERYTHING you do.