Let's see... I have a 16' tagalong (6'6" high), a 16' + 4' (7' high) gooseneck, and a 24' gooseneck, and they are DEFINATELY my prefered transport method. They can do pretty much anything, and with the 24'er they CAN do anything.
The advantages:
1) The trailers are LIGHT. My gooseneck weighs 4100lbs, and the tagalong weighs 2400lbs. They are also usually several thousand dollars cheaper than the equivlant enclosed trailer.
2) The trailers are divided into 8' stalls, with each stall able to fit two horses (full sized) if nessesary. If you need to fit two per stall you can tie them.
3) For mares and foals you can leave them together with the mare tied.
4) For hauling less horses you can leave each horse in their own stall and leave them loose. The horse can balance themselves much better when loose and can also eat and drink on long trips.
5) They can also spend the night on the trailer with a fan if nessesary.
6) The open slats create good air flow, and can be closed if nessesary with plexiglass.
7) You can get trailers built with removable partitions, including slants, making it a VERY versitile trailer for horses or even cargo.
The disadvantages:
1) In an accident there is more risk of injury, but it's not as bad as it appears. There is less danger of entrapment, which is usually more dangerous than the accident itself.
2) Transporting loose horses, while safe, requires a full sized truck. You can't use a smaller truck than you would for the same sized enclosed trailer.
3) The open slats can be dangerous. I'd definately suggest using a fly mask to protect the eyes.
4) For the first few times you travel the horses will have trouble balancing, but they quickly adjust and prefer the freedom.
5) NOT WATERTIGHT
Not even close.
6) There are usually exposed latches in the trailer... not usually a problem, but it's possible. Just be careful making sure that the trailer you choose is safe.
7) The "stock" stock trailer is short; 6'6" is standard, whereas the standard for your average enclosed trailer is 7'. But you can certainly order taller; mine is 7' standard (its a Featherlite HORSE stock trailer).
8) There is a little more roadnoise than the average enclosed trailer, but I've been in some enclosed trailers that were FAR louder than mine!