I'll reiterate, yes they have more power capacity. The load capacity is still the same. A half ton pickup is rated to handle light loads. Yes the engine can do it, however all the drive train components are much smaller. All of it the axles, hubs, drive lines, and u joints are smaller. The shocks aren't valved as stiff, the springs have less "ing", the frame is smaller, and in some cases the tires aren't even rated for the larger capicity.
So yes they've come a long way, but still are built to do the same thing. They just do light loads better than they used to.
I'm sure Ford's planning on coming out with this fancy eco boost in a superduty and all will be fine. Might have to go back to a steel body to put some weight back in it though.
That was the problem when I used to tow a large trailer with my half ton. Either when I got passed or when passing the side draft off the other car hitting the trailer would send me all over the lane. Since I got a one ton truck the wind has no effect at all. Cause it's built for it!