This is half of what I was going to say.For me, having a BUG just gives me options. Carry it in a front pocket and you can put your hands in your pockets if you walk up on questionable individuals. Instead of reacting and going to your main weapon, you can walk with the gun already in your hand.
Typically I carry a single firearm.
If I have a second it's due to a slightly heightened chance to have something go down ( i.e. less safe part of town, craigslist transaction, etc... and if there's more than a slight chance of things getting ugly, I don't go.)
It's also not to backup the primary so much as it is to offer a second option to draw. Most often I do primary right side IWB and secondary inside front left pocket.
That gives options:
1) I can draw with either hand without having to reach across my body.
2) I can casually have hand on firearm if necessary and cut needed time to reach, grip, draw, etc...
3) I have a second should it be needed.
When working a badge carrying job, I always have a secondary. Typically something to compliment the primary (Glock 22/Glock 27)