Considerations for BUG.
Think about what a BUG is used for. Your primary is either down or posession is being "contested". Your backup should be as heavy a hitter as you can handle with one hand. Personally, I'd opt for large caliber, steel frame, something I can control one handed.
As for civilians not needing a backup, what makes anybody in a gunfight different from anybody else in a gunfight? Let's assume I'm not a cop (I'm not) and have a family (I do) and we get in a jam where the only option is to trade ammunition with someone(s).
1) I cannot run away and abandon my family. The aggressor has the opportunity to get as close as he likes (unless I shoot him to the ground).
2) Both my carry guns have proven reliable for several years. What if today is the unluckiest day of my life? It's already pretty rotten if I'm in a gunfight.
3) Beyond the limitations of the primary weapon? What limitations do you think I have regarding the safety of my family? None, zero, nada.
4) Grabbing the primary is cause to draw the backup. If he lets o to grab your backup, shoot him again with the primary.
Personally, I usually carry a fixed blade knife, two inch blade, off side, for a retention tool/backup weapon. The times I do carry a backup gun, it's the second 5" 1911 with two more reloads. I don't go to Indian Country anymore, so that's been a while. And it usually involved a long gun in the trunk, too.