It's simply not necessary and is potentially harmful to the trout. While no trout, large or small, should be manhandled, small trout are particularly susceptible to such mistreatment. They are very delicate little creatures. Mishandling small trout can dramatically increase their post-release mortality rate. They may swim away, apparently healthy, but the damage has been done.
Nets have a tendency to scrape off the protective mucous on a trout's skin.
The mucous allows trout to easily slide through the water, reducing the effort required to get around in its environment. The mucous is also a barrier to bacteria and fungi. To a certain extent, any net will remove some of this mucous leaving the trout susceptible to disease. The actual fabric of the net is an important factor in minimizing the damage to the trout. Rubber nets, like the one pictured, are the best at reducing the negative impacts of netting a trout, but are really only practical when boat fishing.