I'll go with multiple calibers just in case...
In case ammo availability is not what is expected.
Case in point: I had 9mm and .45. Had no interest in .40.
A good used G23 came along in a private sale though and I jumped on it.
I still prefer my G36 (in .45 acp).
But .40 is what most police agencies use and in the future it may come to pass that .40 ammo is the only thing available, at least in that place and time.
June 28, 2012: the day the American republic died.
Uncontrolled, unaccountable government spending + Graduated income-tax = SLAVERY