Exactly. That is why it is extremely dangerous, and illegal, to haul cars around on a 6.5'x12' landscape/utility trailer with a single 3500 pound axle and no brakes. I do not intend to be rude, but I care not what qualifications that poster says they have. Facts are facts, and that is in no way, shape or form even remotely safe or advisable. It actually reminds me of some of the crazy pictures we've all seen in emails with someone hauling their 4 wheeler on the luggage rack of their Taurus or the guy with the wood hitch on the back of an S10 Blazer for a gooseneck trailer. Just because someone has done it, and so far not killed anyone, doesn't mean it's a good idea. On the above mentioned reference, don't take my word for it, call any DOT office or look up any rules governing trailers in any state.
As you said, utility/landscape trailers are made with much lighter materials and the angle framing above the deck is actually needed for support; and that's on utility trailers rated for 7,000 pounds with two axles and brakes. A car trailer has a much stronger frame, generally brakes on both axles, and side pockets that allow you to use 2X4's to drop in and make your own side rails if you like. A 16' trailer would be the absolute minimum I would consider for hauling all but the smallest sub-compact cars. A good quality 16' car trailer that can also be a good utility trailer isn't much more money than a utility trailer anyway.
