Ok, this is my pet peeve
There are three reasons why a power meter being consistent/repeatable is not sufficient
1. The obvious reason is that should your consistent power meter ever break, get lost, or should you decide to switch for whatever reason, your old and new data will no longer match. This is a pretty big problem for anyone using a power meter halfway to it's fullest. Imagine: "How did I pace kona last year, when it went really well? Oh well shit, I don't know any more" or "What CTL was I sitting at when I crushed the state TT in 2010? oh shit, don't know"
2. Imagine the following 4 hypothetical power meters, which are perfectly consistent/repeatable, but inaccurate in the following 4 different ways. Each one will cause problems in evaluating the progress of your training, trying to hit zones in training, or trying to paces races, or field testing:
3. Comparing the overall aeroness of yourself vs others. I've actually used this once to find a setup mistake I made on my P3. Someone the same size as me was going faster on the same watts, with worse equipment. This caused me to look at my fit. It was a new bike, I thought I had replicated a proven good position, I had not. I fixed it, got faster. yay. In other words the ability to do field tests and get an accurate CdA can give you an idea if you are as aero as you should be.
And of course everyone should keep in mind that any power meter that was good at consistency would ALSO be accurate with a simple calibration protocol in software. So in practice it is very rare that you have a repeatible but wrong power meter, and if you do you can at least fix it yourself (with SRM and Quarq) or send it in to be fixed.
Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team Aeroweenie.com -Compendium of Aero Data and Knowledge Freelance sports & outdoors writer Kathryn Hunter