I'm a bit confused---How do you know your LBS guy is right if you didn't get the one with the cadence?

Until the last couple of months, I used a comp w/o cadence feature. I've now found cadence to be a useful feature and I set my display to show it and MPH. At this stage in my riding (started w/ shorter distances a year ago on a hybrid, now on the road bike doing 20-50 mile club rides), it helps as a reminder to spin in the range that works best for me given the conditions. Kinda like having a tachometer when you're driving a car w/ manual transmission.