A benefit of a trailer that hasn't been mentioned is that it might allow you to use an existing bicycle for touring without the need to purchase a beefier bike that can handle the weight of heavy-laden panniers. If you are on a budget, a trailer can be a good option, especially with things like hybrid bikes that already have mountain bike drive trains with low gearing.
I pulled a trailer around behind my cross bike for the first time this summer on a couple of trips. I preferred it much more on the highway trip (Jasper to Calgary) than off-road (Kettle Valley Rail Trail). On rough roads and singletrack, I really noticed the weight of the trailer behind me, as well as the "trailer whip" when dodging rocks, roots, etc. on the bike. On the highway, I only noticed the trailer on fast descents and when standing to climb hills. Packing was a LOT easier than with panniers, and I was definitely more aerodynamic than with panniers. But those were the only tangible advantages. The weight of my cross bike + trailer + bag was about the same as a heavier touring bike + racks + panniers.
I definitely liked travelling on the cross-bike, however. I think I might put on some lightweight racks and ultralight panniers front and rear and try that combination now for both on-road and off-road trips.