Plus, for some reason bicycles loaded with panniers like to fall over when you're no where near them. I'd pull up somewhere for a rest stop, lean my bike against a pole, a tree, whatever, walk away, and crash - I'd look back and it would be on the ground. OK, OK, I guess the bike was lonely, it wanted me to come back! I don't have a kickstand on my bike - maybe it would remain standing if I did... And I did get better at leaning by bike against immoveable objects as my trip progressed.
Another plus for panniers is that if you develop a packing plan you'll always be able to put your hand on exactly what you need whenever you're looking for something. Four panniers = four places to stash stuff. For example, I always had my raingear on the top of the rear right-side pannier. Stove, food, and fuel were in the front left pannier. Tubes and tools in the front right pannier. It's a nice built-in organization technique.
Another possible consideration - in some cases, the weight of the panniers plus racks is slightly less than the weight of a trailer + trailer bag. I don't really think this is a big enough difference though.
Other people I met on the road told me that they liked the trailer over panniers because they could easily detach the trailer from the bike when they wanted to wander around on an unload bike. That's true, it is easy, but it's also easy to pull the panniers off of your bike. My Ortlieb panniers were a cinch to remove from the bike too - just grap the strap/loop that opens the hook over the rack and it's off the bike. So I don't think ease of removal falls on either side of the panniers vs. trailer issue.
Good things about panniers:
- The bike is more compact.
- If you develop a packing strategy, you'll always know where everything is.
Bad things about panniers:
- If you're traveling fully loaded, bike handling is very different. The bike feels heavy, balance point is changed.
- If you're on a trip where you're using other transportation in addition to the bike, you don't have a way of carrying your bike case with you, so you'll have to either ship it on ahead or get a bike box at the end of your trip.