Not willy nilly in the sense of throwing in stuff that has absolutely zero utility, but when I spoke with Bruce Gordon when shopping for my first set of panniers he expressed the belief that many people do tend to take advantage of all available space, at least the first time out. My guess is that such people pack what they "need" and if they are left with additional capacity they say "Well. I still have room so maybe I will take X because it might come in handy. Oh. And Y might be nice to have once in a while, and since I have the room I must not be overpacking." I think a good example of this was the woman on my cross country tour who started out with a Watchman TV and a blow dryer. It was her first tour. Those items (and others) got shipped home at the start of the third day, a few days before we got into the mountains. In fact, at least three people shipped home significant weights on that day.

I am thankful that I have never had that mindset. That doesn't mean I go to great lengths to lighten the load, but I have never felt the need to fill every cubic inch.