This is much like the mousegun discussion...or even the spare mag discussion
The decision is mostly about convenience and probability
What is the probablility of needing a weapon....and how inconvenient is carrying the second weapon/magazine/full-size pistol.
If you truly thought you wer going in harms way...and couldn't carry a rifle....you would likely carry a spare weapon.
But since you have not actually had to defend yourself.....you convince yourself (often incrementally) that a second weapon is not needed