This horse (no pun intended) has been beaten to death many times before. The permit and registration system in the GSMNP has been in effect I believe since 2012. It came about because of the overcrowding at shelters and numerous people stealth camping inside the park. Even now the ATC is asking, not requiring, people to schedule their departure date on their website, in order to avoid overcrowding along the whole trail. Baxter State Park has instituted a permit system to go on the trail up to Mt. Katahdin. All three examples are caused by
overcrowding. Some people, myself included, put a lot of the blame on a movie about two friends hiking the AT. It is felt that this brought about a huge amount of attention to the trail that resulted in certain people with no previous hiking experience and that don't have a clue how difficult a thru hike is to complete. They go out and buy equipment without researching it and usually end up bringing way too much stuff and or the wrong type of gear. You will find it on the stairs on the approach trail and along the trail north of Springer Mountain just lying on the ground. They have no concept of LNT principles.
Now getting back to the Smokies, I will tell you up front that I personally detest shelters. Between the mouse poop, food stains, stains left by people who got sick (read puked) and the stinking, dirty, farting, belching, snoring hikers (nothing personal), just let me camp away from the shelter. I also don't want mice running back and forth over me all night or bugs that can give me certain diseases. I am the one that will setup in a shelter, but will gladly give up my spot to a late arriving hiker.
Some have mention the damage to the trees and and the ground that hammock hangers cause. A veteran hanger carefully chooses their spot. Live trees, no dead ones, that are the correct diameter (12 inch minimum for me) as well as the correct distance apart (15 feet minimum though I prefer 18 feet when I can find it.). Are there any widow makers overhead (dead branches that can fall on me). The correct distance from water sources (200 feet recommended). Ground underneath, is it the point of not being able to be reclaimed? If it is use it, but if there a tent camper there and it is a level area, leave it for the them, as a hammock can hang over slanted ground. If the area is capable of being reclaimed, leave it alone and find somewhere to hang. As a last resort we use virgin areas. After such use, we will do our best to restore it to it previous condition. Chances are low that another hiker will use that exact same spot.
The physics of hammock hanging is complex and should be left to the Sheldon Coopers of the world. Derek Hansen has a neat
calculator on his website that shows you how changes in one aspect of the hang can effect to rest of the dynamics. It also shows the shear force applied to the tree at the strap height.