Small revolvers, in particular the light weight variants, prove versatile secondary weapons. My bug (backup gun) has been a Ruger LCR for some years now. The truth is, I usually do not carry a bug on a daily basis, but I sometimes carry a bug if traveling. The bug also serves other purposes for myself: it is my home carry gun when I take off my primary handgun in the evenings and it is my vacation gun when dressed in beach cloths. A small revolver is easy to carry and it is a gun you can carry when nothing else chambered in a service round will work.
While the small revolver has filled the BUG and deep concealment role for decades it is not the only option today. The new crop of small single-stack autos chambered in 9mm work as BUGs for a lot of people. Guns like the Smith and Wesson M&P Shield or the Glock 43 are very small, although I find their auto-loader shape more difficult to conceal than the rounded small revolver. The autos that I find to truly be BUG in function are the tiny pocket autos chambered in 380 acp. Guns like the Ruger LCP and the Smith and Wesson Bodyguard are truly small and concealable but I have reservations concerning their reliability. Many have good luck with these, but many do not. The small revolver, while slightly larger, proves infinitely more reliable, but if you have a small auto that is reliable than certainly it can serve you well.
A defensive firearms lineup is simply not complete without a BUG. Even if you never carry it as a second gun the vast majority of concealed carriers will face times when they can’t carry their usual full-size handgun due to environment or dress requirements. For such occasions an exceedingly small and concealable gun is not a replacement for your serious pistol, but it is a much better alternative to no gun at all. The BUG also works great as a home carry gun or a vacation gun. A snubby or pocket pistol in your cargo shorts while at the beach is much better to you than your full-size pistol sitting in the glove box of the car.
Beyond the pre-requisite of being very small, I believe that most people are best served by a BUG that is also very light in weight. For example, my Ruger SP101, which is a much nicer shooting gun than is the Ruger LCR, does not serve a BUG role very well even though it is only slightly larger than the LCR. The reason is the weight. It is literally twice as heavy as the feathery LCR and while it proves no issue when worn on the hip I find it out of the equation for pocket or ankle carry. Versatility of carry mode is essential for the BUG and light weight is vital to this.
If you don’t currently have a backup gun then I truly suggest looking into a small, highly concealable, and light weight revolver or pocket auto. Remaining armed at all times possible is a good thing and a BUG/deep concealment gun will serve you well on many occasions where your full-size pistol will get left behind.