4x4, that's a valid question and one that's been asked here several times over the years. In a nutshell, there are just too many variables to make a blanket statement as to whether an anti-sway mechanism is "required" or even a good idea.
If your rig tows fine without anti-sway and you are comfortable driving it, then I would say you don't need one. I'm with Doug33, I think it is good insurance.
We bought the Hensley when we got our second TT, a 34' Sunnybrook bunkhouse. We loved the floor plan with 4 bunks for the kids and the storage was amazing! The entire area under the bunks was clear storage:
In this pic you are looking in through the curb side access door, the one across is the other side of the TT and the one to the left is the rear of the TT. So we had this giant 6'x8' storage space that got filled with tables, chairs, generator, BBQ, fire pit, etc., etc. The only problem was all that storage was at the very back of the TT, well behind the axles....so you know what that did to the tongue weight. Even with an extra battery and stuff under the front queen bed, tongue weight was barely 10%. Point being that NOT every TT made today is designed with good or even acceptable weight/balance.
What we found with the Hensley was that towing was much more comfortable...the 4-bar linkage is not sensitive to tongue weight so loading/weight/balance on the TT were not as critical as they were before. Even though we were within all of our ratings, the TT scaled at 8500 lbs and the burb at 8600 lbs, and the burb has a relatively short 130" wheelbase to control a long TT. (GVWR=8600, Tow=12000, CGVWR=19000)
We found that situations on the road had zero affect on the trailer, things like uneven pavement in construction zones, high cross winds, wet roads, passing semis, etc. I can tow the TT all day at 65-70 mph in any conditions with two fingers on the wheel, just like driving with no TT attached. I won't say "you don't know it's back there" because obviously that is BS...of course I know it's back there, but what's different is that I don't have to worry about what it's doing. With the Hensley, I know that the TT isn't doing anything but following the truck in a straight line.
I even got my wife to tow a few hours on the road, and she was able to handle the rig just fine, having never towed a trailer behind anything in her life.
I know folks on the forum here like to call hitches like the Hensley a "band-aid" that simply masks the conditions of an unsafe set up. Fact is, we don't "need" a Hensley hitch, we could tow our TT just fine with a conventional hitch. Of course we don't need a TT either....we chose to buy one because we like to camp. Likewise, we chose to buy our Hensley hitch, because for us it gives us peace of mind knowing that hitch has the TT under control at all times and in all conditions. For us, going camping is about relaxing, and the Hensley makes the tow there smooth and uneventful.
I'm not suggesting that everyone should run out and buy a Hensley, just trying to explain why I bought one. If you're happy with how your rig tows today, then I would say you're all set.
There are only 3 hitches whose design truly prevents trailer way: the Hensley Arrow, the Pro-Pride 3P and the PullRite. These hitches definitely work as advertised, but that's not to say there isn't a certain amount of marketing hype in anything you buy that's RV-related.
4x4 also makes a good point about complacency, and that is always a risk.
