I've been thinking about getting a cadence thingy for my Garmin, largely because I'm curious and I like gadgets.
I come at things from a slightly different perspective: My objective is always economy, not performance. That is I want to go as far as I can, and is speed largely irrelevant apart from how it contributes to my overall economy. I don't want to be dead on my feet after 100 miles. So I need some modicum of speed or sheer time in the saddle will take its toll before I get to my destination.
Now, my homespun wisdom is that I'm better off in a higher rather than lower cadence to maximise economy (within reason - not spinning my bottom gear on the flat). I don't know if that's right, but it's a kind of gut-feel thing. If I grind hard for the first 50 miles I feel I'm more likely to fatigue my muscles and probably run out of steam after 80 (say).
I also feel that the use of a generally higher cadence may be easier on my knees, which are a bit dodgy.
Whether the above is correct I have no idea - it's just gut feel. "Change down not up" is one of my private rules, along with "no pedalling on downhills above 25 km/h", "no hard accelerations - work up through the gears slowly", and "sit, don't stand" with which I hope to ride efficiently. All probably flawed ideas, but I manage my long rides OK with them.