"The closest it comes (not very) is, "If 1 were admitted as a prime, these two presentations would be considered different factorizations of 15 into prime numbers, so the statement of that theorem would have to be modified." So what?"
Many proofs depend on the unique factorization of primes, many directly and many many more indirectly. It is a trivial mechanical modification to them to deal with the non-uniqueness of a prime factorization if you admit 1 as a prime number by explicitly taking that case and saying it doesn't affect this case. So in that sense, no, it's not important.
Except... you've now taken numerous proofs and made them longer... and for what? There's no didactic advantage. There's no proof that is made easier by letting 1 be prime. What's the advantage of adding all these special cases? None.
And that's the real reason. In the end, "by definition" is the only justification, and the reason we choose the definition is that it works the best. Unlike 0 to the power of 0 where there's at least a bit of argument to be had (though the overwhelming preponderance is for it to be 1), there's no reason to put 1 in the set of prime numbers. Even if you don't personally consider it a "lot" of evidence, it's still entirely one sided.
Many proofs depend on the unique factorization of primes, many directly and many many more indirectly. It is a trivial mechanical modification to them to deal with the non-uniqueness of a prime factorization if you admit 1 as a prime number by explicitly taking that case and saying it doesn't affect this case. So in that sense, no, it's not important.
Except... you've now taken numerous proofs and made them longer... and for what? There's no didactic advantage. There's no proof that is made easier by letting 1 be prime. What's the advantage of adding all these special cases? None.
And that's the real reason. In the end, "by definition" is the only justification, and the reason we choose the definition is that it works the best. Unlike 0 to the power of 0 where there's at least a bit of argument to be had (though the overwhelming preponderance is for it to be 1), there's no reason to put 1 in the set of prime numbers. Even if you don't personally consider it a "lot" of evidence, it's still entirely one sided.