Do you mean EDC or in the PSK? I will address each.

EDC: Sidearm. I have never had to use my sidearm in an urban situation, luckily. A couple times I was glad I had it as things could have gone differently. I'll note that I have had to use my sidearm in a wilderness setting. EDC-wise, everything else that I actually carry gets used with some regularity.

Wilderness: I generally have a very well stocked PSK and I have a few options. For state parks and places where the odds of getting lost or stuck are minimal my kit is fairly minimal. Then I have an intermediate kit built around a water bottle bag. In theory it contains enough to survive an unplanned night or two in the wilderness, although it wouldn't necessarily be comfortable. Lastly I have a pretty comprehensive PSK/day bag that I built around a Mystery Ranch 2 Day Assault pack. At 27 liters it's still easy to carry but very close to a camping pack. With it I'm be fairly comfortable down to 40 or lower even without a fire, and with a fire I'd be pretty good in anything short of a very bad winter night.

I will confess to having to resort to a lot of the kit in one instance that I recounted here at ETS. Even then there are a few thing I didn't use. As Teacher mentioned I've never used my emergency whistle, nor have I ever needed my signal mirror. So far I've never used the space-bag/bivvy outside of just testing it. Certain items in the FAK have never been required in the field, either. Lastly I have never used the sail needle or fishing stuff that seems obligatory in PSKs. Truth be told I'm terrible at sewing so if I did need the needle it would be a mess. The fishing stuff is only there because it's part of an otherwise-useful kit I cannibalized to build my homebrew kits. I suppose there's little harm in having it as the weight is low but I probably wouldn't go out and buy more if it got lost.

Of course, just because I haven't used it yet doesn't mean I never will. I've played with mylar/space blankets for a couple of decades but never actually used one in a tough spot until this spring! Luckily I try to experiment and practice with all the stuff I carry so I don't have to figure it out on the fly.
“You can tell whether a man is clever by his answers. You can tell whether a man is wise by his questions.” Naguib Mahfouz