I like pedometers - always have. Still have one of the old mechanical ones from pre-teen days. On various "walking" sites, like "walking.about" you can find reviews of what's available now. My current one is a Sportline 342. I think the company has changed its lineup, but I still see this one at Walmart and such. Anyhow, its light, and cheap, and small. Its numbers face up when its on your belt, so you can just glance down and read it. Amazing how many brands/models have a less convenient layout. I use it on all my recreational/training hikes, as its nice to set goals and go to bed knowing I did 8 miles this evening, or 50 for the week, or whatever. Pedometers really only count your steps, and you set it for your average stride and it then derives the mileage from that. Obviously if you're going up a steep hill and shorten up your stride, the pedometer will "think" you've gone farther than you have. Despite that, mine is quite accurate. I've checked it on a route that goes around a 640 acre "section" which is of course a mile on a side. The car odometer read it as one mile on a side, and the pedometer did too, although it varied about 5-7% each time I walked it. Surprisingly, gps varies more, about 10%. After seeing this question, I walked a 2 mile route 3 times - the pedometer read 1.91, 2.03, and 1.98. The GPS read 2.13, 1.99 and 1.87.
Anyhow, they're fun...I suppose not really NECESSARY. Your watch will tell you when its time to take a break, and lots of experience trying to make campsites by nightfall that are "only" 6 miles distant, will give you a pretty good idea how long it will take to get somewhere. Nevertheless I like mine. BTW some health-oriented groups will give away cheap pedometers, to encourage people to lose weight and stuff. The "10,000 step" program is the best known. I say, pedometers are neat, and fun, and cheap, so get one and let the wife use it.