What matters more than anything is that your particular powermeter is able to produce repeatable numbers day-in/day-out. 250 watts today should be 250 watts tomorrow. It doesn't even matter if your powermeter's 250 watts is the same as someone else's 260 watts. Its the relationship of your numbers to you that is important and that allows you, and your coach if you have one, to plan or alter training to produce increased performance.
That's true only so long as you don't have to worry about an external reality. It's like saying you don't need to worry about the accuracy of your car's speedometer as long as it's repeatable -- try that argument on the judge when the police say you were speeding. The most important thing about a power meter is the quality of the data it produces. Everything else is a feature. Being able to check the accuracy and precision of your power meter is key. One might even say a "9 key."
I'm not following your analogy to a car's speedometer....one is faulty equipment that leads to you breaking the law and is compared against an objective standard. The other is piece of bicycle equipment that only needs to be compared against itself.
As TriBriGuy said, the key is consistency and repeatability. If the PM reads 250 watts for a given input, as long as it always reads 250w for the same input, you are fine in regards to training.
Note the emphasis. Whether the wattage reported is the actual wattage is almost immaterial. You are training against YOUR values only. As long as your measurements are consistent and repeatable, then you can train properly with those measurements. Everything is still a percentage of those readings.
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