My first bike computer had a cadence on it and I really did like it. I had to replace it a couple of years ago the rain in the northwest did it in. My new on is wireless which I love but no cadence and I do miss having it. I say if you can afford to get one do it you won't regret it.
As far as for shifting that's more of a feel thing, cadence just lets you know or helps you get into a good cycling zone and helps you push yourself harder, it is a good training device.
Happy pedaling!
Life is like a 10 speed bike, we all have gears we never use.
Charles Schultz
"The bicycle is just as good company as most husbands and, when it gets old and shabby, a woman can dispose of it and get a new one without shocking the entire community." — Ann Strong, Minneapolis Tribune, 1895