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.
_________________________
“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman