True, PK, to each his own, but let's look at one example...
Went to Firearms Academy of Seattle in August, for a class with Massad Ayoob. Pax (Kathy Jackson) works there; she recommended that I bring a spare, just in case, because failures have been known to happen.
FAS is a nice facility, but they are over a 30 mile drive to Olympia, so finding an open shop, with required parts, could prove a challenge. Buying another handgun, when not a Washington resident, would be illegal and effectively impossible.
So... airfare from ATL to SEA (round trip) plus cost of hotel for five days plus cost of rental car for five days plus cost of ammo shipped to FAS (they don't stock ammo there)... plus $800 for course... I'd say I had about $3000 into the trip.
I was using a S&W M&P45 as primary; it seemed to me that $550ish for a second M&P (compact) was not a bad price to pay, to ensure that I wouldn't screw up a $3000 trip; besides, having a second gun with similar handling characteristics, manual of arms, and ammo, plus one-way magazine interchangeability did not seem like a bad thing.