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USB charging amperages can vary, from less then the mentioned 450mA to little less then the 2A AC charger, I'm certain Sony took that into consideration.
The lens analogy is a bad one, the quality of the lens directly affects the intended purpose of the camera. As long as the charger isn't completely out of range then it has absolutely no effect on the intended purpose of the reader.
I think the habits of the user are going to have an immensely greater impact on the lifespan of the battery. The storage conditions, times between charging, whether it is charged regularly without being drained, etc. Most likely the generic chargers are made in the same factory as the Sony original after-hours, just unbranded. A -very- common practice.
But I absolutely agree with you that the charger should come with it.
-MJ
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