There are some states where riding in a TT IS legal, Indiana being one of them. On a more practical side, is it a "wise" or a "smart" thing to do? No!
Only once did I allow someone to ride in the TT on the road. We were on vacation returning home, Trailer in tow. My mother was visiting my sister and we thought it would be nice to stop in for a few moments before and see them before the last 10 miles to home.
When we got there, my sister told me Mom was horribly sick, she thought Mom was having a heart-attack, but absolutely refused to go to the hospital or allow for an ambulance. (You just don't understand my mother ... it's a very complicated story)...
Well, she was definitely in a very bad situation and absolutely refused an ambulance, so we convinced her to go into the trailer. Yelling and screaming the whole way (it was quite a scene), we got her inside. I slammed the door, jumped in the truck and headed for the hospital. Once at the hospital, they were waiting, came inside the camper and drug her A$$ out yelling and screaming the whole way! She was in the hospital for 2 weeks, and to this day still denies she had heart problems!
It was nice to know that I was not breaking any Indiana laws by having my mother and sister ride in that trailer! So you know, there may be sometimes when it is ABSOLUTELY necessary. But, to make it a common practice is simply not very wise.