When I was in school I heard something similar about math definitions. I didn't hear "Mathematicians are always right" or "because it is defined to be even"; Rather, in response to questions of "why is [this] defined to be [that]" one professor simply said "because it is useful." Usually this meant "because it means this property of this system holds true if [this] is defined to be [that]." For example, the first two Fibonacci numbers, fib(0) and fib(1), are defined as 0 and 1 because it makes the Fibonacci sequence work. They are 0 and 1 because it is useful as far as calculation of the sequence. I am pretty sure that in practical applications (por ejemplo, examining the fib sequence in nature) fib(0) has no use.
