Also worth thinking about - Quote Taken from a gents on the web.
One way to look at it, is that when you consider the role of the shock, it is to damp oscillations of the suspension. The oscillations occur when the spring cycles more than one time, i.e. bounces. Now, consider that on most trailers, a high spring rate is employed to carry heavy loads; then, when a trailer is loaded, the load doesn't move, only the wheel moves (ideally). The mass of the load actually dampens the movement of the wheel. This is obvious if you consider the following: remove the rear leaf springs for a 1 ton truck, and put them on a stock samurai. The springs will not really move very much, and its almost like there is no suspension. This is the unloaded trailer. Then, put the samurai springs on the truck; here the springs will compress and the truck will not really move at all (provided the truck hasn't maxed the suspension to the bump stops). This is the loaded trailer.
Now, the reason the shock would be useless in most situations, is that on the unloaded trailer, the suspension doesn't move, therefore there is no movement to damp, and the shock just sits there (i.e. no cycling of the suspension).
On the loaded trailer, the load acts as the damping force for the system, and a shock would be redundant.
Keep in mind also, that ride quality is not a factor. In that respect, a trailer suspensions only two duties are to carry a load without compressing completely, and to allow the driver to maintain control of the trailer. Cost is also a factor. Finally, realize that a suspension where shocks play pivotal role (i.e. a vehicle) is not able to carry much weight. Compare a truck, even a 1 ton, to a trailer that easily haul 5000lbs. Then think how much more it would cost to make that trailer suspension feel and perform like the trucks in terms of ride quality.
