...So, while accuracy is important, consistency is crucial.
If you have to replace your PM at some point do you want the replacement to produce different power numbers than your old PM?
If you decide to buy a second PM for another bike/wheel do you care if it produces different numbers than your first PM?
If you hire a coach who wants to compare your power numbers to other clients will he want your PM to produce numbers consistent with theirs?
I'd venture to guess that for most people accuracy is crucial.
If you replace your PM with a PM of the same make, most likely any differences will be smaller than the differences in your sustainable power day to day, so hardly even noticeable. If you replace the PM with a PM of a different make, it is easy to account for any differences in output, e.g. PT against SRM...
I have indeed two PM's i train and race with (Powertaps). I keep any eye on their calibration routinely, so the above applies here as well. Differences between the two PT's are not noticeable, and (if they exist at all) definitely much smaller than the variance in my power day to day.
Why would a coach want to compare my numbers to anybody else's numbers? Makes no sense.
OTOH: When my PT tells me, that my FTP has improved by 50 watts during the last 12 months, i want that information to be absolutely reliable. For effective and successful training with a PM, it is really not necessary for the PM to be accurate down to a watt or two, but to be consistent, believe me.