Based on the number of people in America, and the number of crimes committed against people, including the number of these crimes committed among ciminals against each other, there is (very conservatively) something less than a 5% probablity you will ever need your main gun.
SOURCES: US Bureau of Justice Statistics (2004), Federal Bureau of Investigation, (2005)
Now let's make some more assumptions. What are the odds your main gun will not work? What are the odds that your primary arm/hand will be injured at just the wrong time? What are the odds that you will run your primary sidearm empty in a gun fight? Maybe 1 out of 100 for each? Well that's 3%, so 3% of 5% means the probablitity of needing a backup is approximately 15 times per 100,000. Then you have to make a guess about whether the caliber of your backup will really matter, and the probablility starts to get pretty small.
But in spite of that I always carry an off side backup when I go downtown. Sometimes a .380, sometimes a .25, depends on what I am wearing and where I am going. I don't carry a backup when I go up to the Sears hardware store or Walmart though. As far as worrying about what THR posters think about the .25, or the .380, who cares. By actual count and survey99.99875% of posters on THR have never used a firearm for self defense. I've only "almost" used mine once since I got out of the Navy in 1979, and I didn't have to pull the trigger, just cleared the gun and the BG's took off.