Right, but apart from old as hell bikes (like the beater road bike I tried to buy on Craigslist to use as a commuter), there's 1 standard for how a pedal attaches to your bike (I am pretending side mount pedals
are not a real thing here), whether it's a beach cruiser, MTB, CX, road, TT, track, fat bike, etc. Powertap, while it can be built into any wheel, often doesn't come in many wheels such as any Zipps and probably other brands I don't feel like thinking about at the moment. As others mentioned, though, there are different types of wheels such that a Powertap wheelset cannot be transferred between bikes, and quite a few triathletes, particularly smaller women, may have a TT bike with 650s and a road bike with 700; it's really not that uncommon. Others, though I'm not sure I would think it's a good idea, may want to occasionally take them out on a MTB and hope they don't clip one of the pedal pods on a root or a rock. Your road Powertap isn't going to be able to fit that.
I'd venture to guess that you are correct that universal adoption of Powertap would cause fewer equipment changes than universal adoption of Keo-compatible Vectors, but the Keo-Vectors can go on ANY bike (in addition to the fact that Garmin has indicated that it wants to create other pedal platforms and that the Keos are just a starting point). Find me a Powertap that does that.
On another note, I now think it would be fun to see how putting Vectors on an electric bike would make for some funny power readings. Because, you know, we can do that. We can't put an SRM or Quarq or Powertap on one, though. :)
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