In sort of an interesting twist on that theme -- and it runs against the grain of all the womanly advice I've gotten over the years -- I don't carry a spare set, nor do I keep a spare set in a desk-drawer at work.
I used to, years ago, but perhaps astonishingly I've gotten vastly more graceful than I once was when it never mattered if I blundered into something. This is benefit enough, mainly because I damage the world around me less than I once did, but that it also helps keep the clothing bill down is just an added bennie.
I figure I can get 15 to 20 wearings out of a typical pair of sheers, and do not believe I've purchased new stock in about 3 years. There are some secrets to be shared in this venue, most of which revolve around materials science. Contrary to laundering advice, I do not hand wash my legwear; that all goes into a mesh bag and into the load of wash that has compatible colours (sheers go in with the whites). What I don't do is throw them in the dryer, I hang them up to dry after carefully making sure that there's no twisting going on (this is especially critical for patterned or cable-knit designs). I also take great pains when donning the things to not twist them, nor apply any sharp "fingertip" force to them, preferring sliding forces instead of "pulling" forces (in other words, fingertips are extended when pulling them up rather than curled inwards).
My late ex- was astonished with the results because she had such rotten luck. I've actually gotten better over the years with the longevity.