I don’t see the point in having cadence. What are you going to do with the knowledge that you’re pedalling at 88 RPM? (Which is anyway knowledge acquirable by counting while looking at a clock for 15 seconds.) It’s data for the sake of data. There’s quite enough of that in my life at the tail end of 2017.
Because I am a beginner and I find that having such info can help me with improving my pedal efficiency. I also take the approach that I'd rather have the data and not use it, than want the data and not have it. Also being the geek I am, managing datasets is kinda what I do for a living these days. Data is good, Data is useful, I like Data.
But speed derived from wheel RPM is useful, because GPS receivers, while very accurate over a long time scale, are poor over a split second at the slow speed of a bicycle. Consequently, there’s a lot of data smoothing in the short term, leading to uncertainty about your instantaneous speed even in open terrain. For me, this becomes a minor problem when, for example, taking a pull at the front of a group and wishing to hold a steady pace.
Just how short is your turn on the front?
There’s also the problem of the GPS not knowing when you’re stopped.
Unfortunately the accelerometer-based wheel-speed sensors are large, ugly, wireless devices that strap onto the hub (a beautiful part of the bicycle that I prefer to leave visible) and need their battery attended to.
Battery attention is the only downside I see. Yes the hub can be a work of art, and strapping something to it can detract from that, but ultimately while my bike is IMHO a thing of beauty, she is also a thing of utility. Having a sensor helps me ride her better, so the cost is worth it.
And anyway, I won't notice under the You Don't Want To Know™[1].
J
[1]Not entirely sure what it is or where it comes from, but it seems to have a near invisible coating on the streets of Amsterdam, yet the simple passage of a bike over it is enough for it to coat EVERYTHING with a thick layer that you can never quite get clean again. It's also the composition of the bottom of canals, under the 3m of water, there's 3m of bikes, then it's just You Don't Want To Know™ all the way down (until you get to the turtle...)