Nice thing about it is that you can also carry furniture and mattresses and all sorts of other stuff on it. 
Thanks Mary.
Another thing I like about the flatbed concept is that if you have cradles on there for your boat and you miss the cradles when you are putting the boat on the trailer, you don't have to worry about it being damaged by the metal superstructure of the trailer. If it misses the cradles, it just goes onto a nice soft carpeted flat surface. And there are no sharp metal things for it to get damaged by when you slide it on or off the trailer.
These are the primary reasons we chose to go with alloy for our Tilt trailer build. Welding alloy will ruin the tempering and create a lose of strength ~50% in the heated region. Welds typically fail before the section does, the opposite of a steel weld.
We also went with a Torflex suspension axle...no springs to rust out!
Been running it for 2 years now...done 2 trips to Houston & back (~3000 miles each trip).
Here's a webpage with more details on the design:
Trailex lifetime? My 1978 Trailex is under my new A cat. It has seen 28 years of regular racing (TheMightyHobie18 and H20) and STILL looks GOOOOD despite salty spray and beach-drift mist. Note- it was always washed on Sunday night after each of many ocean regattas. And my back says- thank you very much! 
PS: I towed it with a Hobie 18 to the lake a couple of times with my 40 horse VW Karmann Ghia- 53 mph on the straight!
I have seen Trailex trailers outlast the boats they tow. When I bought it I was told to expect at least 50 years out of it if I took care of it. I will have to replace the wheels, bearings, springs, and axel before I do anything to the frame.