I mostly agree, but would say, "Assuming the seller isn't stupid or you have information they do not...", which is sometimes the case. E.g., you know you'll be subjecting the device to conditions that contribute to failure but don't void the warranty.
For example, I recently bought a comprehensive SquareTrade warranty for my Roomba because they priced it as a "vacuum cleaner", when in fact I know that the Roomba (a) an immature technology compared to most vacuums and (b) more likely to fall or be knocked down the stairs. I did the math and decided the odds of this thing dying or being killed in the first three years were greater than the warranty/purchase price ratio.
For example, I recently bought a comprehensive SquareTrade warranty for my Roomba because they priced it as a "vacuum cleaner", when in fact I know that the Roomba (a) an immature technology compared to most vacuums and (b) more likely to fall or be knocked down the stairs. I did the math and decided the odds of this thing dying or being killed in the first three years were greater than the warranty/purchase price ratio.