I've done both ways: panniers vs. a Bike Friday style trailer system. On the whole, I think I still prefer panniers for lower overall weight and maneuverability, but the trailer definitely had some bonuses. One questionable bonus, as you mention, is that you can toss stuff into that trailer and not dwell on the extra weight/bulk. When going on level ground, it's fantastic, almost like riding unloaded. When going uphill or over rough terrain, not so fantastic. I tend to travel with items of dubious use to the average tourer, so I can hardly claim any experience in regards to the joys of ultralight touring, but I have found that getting my gear from four panniers down to two has made for more pleasant riding. And I have found that the comparative ease with which you can add gear to the trailer can be a temptation.

One issue for me is that often my trips are as much about camping as they are about riding. My latest trip had me carrying items that were more focused on the camp site, like my camp chair, cooler, frisbee, extra non-biking clothes. All made possible by the trailer (although going back to four panniers would have done it as well). I'm not sorry to have brought those items because I ended up using them, enjoying the use of them, and, in the long run, I probably spent as much time or more at camp than I did on the bike. But if I were going on a trip that was primarily a bike tour, I would want to pare down my gear quite a bit.

So I would say to asses your needs and goals, and make your decisions that way. If you were going to share a tent with someone, or spend rainy days spent entirely in the tent, then a larger tent might merit the extra weight, but if it's just so you can bunk with your trailer, it's probably not worth it. For me, finding a small, portable tent (I sleep in a hammock tent now) was the thing that led me back to bike camping. I don't know what it would take to persuade me to take a "real" tent. Probably it would take a sudden interest on the part of my wife in accompanying me.

So, I am in no position to argue against "unnecessary" items in your trailer, but you will pay for every pound you add, so make sure your extra gear provides a real benefit in your trip rather than using the "it fits, so I might as well take it" line of reasoning.