The *need* for a trailer is a very subjective concept, as it's influenced by people's budgets, comfort levels, type of trips, routes, and even geographical location. Having used both a trailer and a roof rack (and sometimes both), I can tell you that it's the terrain that really defines it for me. If you're going to be taking very rough trails, or very technical ones with tight switch backs or the possibility of having to backtrack (like when exploring old mining roads in AZ), you probably want to leave the trailer at home and just use a roof rack. On the flip side, if you're going on a very long trip with more than one passenger (besides the driver), or plan to visit a remote location and then use that as a base camp, or if your expedition rig is also your daily driver and you tend to go on trips on short notice, a trailer makes a lot more sense. Another situation where I see the trailer as an advantage is on long expeditions where you spend most of your day driving, and the trailer becomes a self-contained chuck wagon and "main campsite" that can be quickly prepped with minimal unloading of the rig itself.
I agree that traveling with less gear is probably the ultimate solution, but for some of us (and especially our spouses), rolling up your toothbrush and two Powerbars inside a t-shirt zip-tied to the rollcage isn't an option, even more so if you like to travel/camp with your family.
I think it was Kurt that mentioned this a while back, and I'll reinforce it. Before I had my trailer, I used to come home from a camping trip facing having to unload and store all of my gear in various places. Then, on the next trip, I had to go find all the gear in the various places (now forgotten), then load up the truck and hope that I didn't forget something important. Now, my trailer is a self-contained unit, I keep all of my gear neatly stowed in it, accounted for and centralized, and the only thing I need to add is the food box and other consummables, and I'm ready to go. That, my fellow expedition enthusiasts, is priceless!
Granted, I'm not going to go on some highly technical trail with the trailer in tow, but I built mine to follow the Land Cruiser and it's as heavy duty as the truck, I am confident that I could take this trailer around the world with minimum hassle (although I wouldn't want to be on muddy or sandy tracks with it).
For reference, it's a converted ex-military M416 trailer (with extensive modifications) that I tow with my 60 series Land Cruiser.