So, so glad to see this response.. there is often an advantage when a pickup can go from a weaker to stronger character and tapping is one of the most useful approaches, but often ignored..
As Jeremy mentions, they are a pain to wind and switching can also be a pain. However the tone shift can be incredibly useful and mass production could overcome some of the challenges and costs that low run winders encounter.
And there is an extremely good example of an excellent production pick up.. the vox coaxe is a coil within coil design for noise reduction and both coils are tapped to provide a regular and overwound timber.
I've had a couple of the vox guitars that include the coaxe pickups and I built a guitar around a set because of their great flexibility. The tapped tone is very similar to a clear and slightly underwound p90 and the full wire version is a pretty snarly mid-ranged p90 tone and it's noise free either way.
I have posted the following article many times.. it is a in-depth explanation by vox engineering and I continue to wonder if Seymour could wind something like this without infringing patents..
Vox only wound these for their guitars so there is very little knowledge in the real world, but if SD ran with a similar concept, I believe it could become very successful..
http://www.planetz.com/vox-coaxe-interview-with-vox-rd/
Also, vox created a switch dedicated to flipping the taps simultaneously on both coils, however, I was able to do the same thing with a super switch.